Nobody's buying CDs anymore, but everybody's going to shows.

With music sales continuing to slide, music acts are headed out on the road in the hopes of earning their rent money at the ticket window and T-shirt stand - qualifying 2008 as the busiest concert summer in memory.

The list of recommended shows that follows is divided into 10 events performed in the open air, and 10 going on inside, where air-conditioning, theoretically, will be available.

By no means is either list meant to be encyclopedic - hundreds of acts are touring this summer. I could go on highlighting ad nauseam, with other outdoor shows of note including Pearl Jam on June 19 and 20 at the Susquehanna Bank Center and My Morning Jacket on Sept. 5 at Festival Pier, or roof-over-your-head shows such as Chrisette Michelle and Raheem DeVaughn at the House of Blues on June 26 and Wilco at the Wilmington Grand Opera House on Aug. 10.

Outside

The Roots Picnic. This is a one-day-only exercise in inspired concert programming. Philadelphia hip-hop leaders of the pack the Roots headline, with a superb undercard that includes hip-pop odd couple Gnarls Barkley, electro-rock maven Santogold, retro-soul belter Sharon Jones and the Dap-Kings, alt-rappers the Cool Kids, Philadelphia DJ-producer Diplo, and rising jazz bassist Esperanza Spalding. Should make for an idyllic day on the Delaware Avenue tarmac. June 7 at the Festival Pier.

R.E.M. Ever since drummer Bill Berry walked away in 1996, the three remaining members of R.E.M. have been making music of diminishing returns. That pattern is reversed on Accelerate, the new guitar-driven album in which Michael Stipe, Peter Buck and Mike Mills take their album title to heart and reclaim their relevance in the process. The openers are Modest Mouse and the National, two bands who grew up in the alt-rock environment R.E.M. helped to create. June 18 at the Mann.

Maze featuring Frankie Beverly, with Chaka Khan. With the Robin Hood Dell East undergoing repairs, old school R&B in Fairmount Park is at a premium this summer. Philadelphia native and a seasonal perennial Frankie Beverly will carry the load, reliably delivering his "Joy & Pain" soulful disquisition on the ups and downs of everyday life. He'll be joined by Chaka Khan, who returned to form with last year's swaggering Funk This, and will be back in town to receive an R&B Foundation trophy on Sept. 9. June 20 at the Mann.

Popped! Festival. In its second year, the Philadelphia Popped! Festival takes to the streets near Drexel University with an estimable all-day Saturday, June 21, lineup of indie bands including Ivy League Afro-poppers Vampire Weekend, gypsy punks Gogol Bordello, and girl-boy Brit electro-poppers the Ting Tings. The fest begins June 20 with legendary rapper Slick Rick at the Trocadero, and closes shop June 22 with an all-ages date at the World Cafe Live with Daniel Johnston, plus Philadelphia's own Capitol Years, Tickley Feather, and Cheers Elephant. June 20-22 at various venues.

Coldplay. Not counting a free show at Madison Square Garden the week before, big deal lite-rock foursome Coldplay opens its summer U.S. tour in Philadelphia. Viva La Vida, or Death and All His Friends, the fourth full-length album by the band led by Gwyneth Paltrow's scruffy, piano-playing hubby Chris Martin, takes its name from a Frida Kahlo still life painting of watermelons. More to the point, musically, it's produced by Brian Eno, the U2-connected experimenter who, if the album's first single "Violet Hill" is any indication, just might have given Coldplay a much needed kick in the pants. June 29 at the Wachovia Center.

Paul Green School of Rock Festival. The ever-expanding empire of the Philadelphia-birthed Paul Green School of Rock takes over the Festival Pier for two days this summer. New Wave-era rockers Devo - famous for "Whip It" and wearing flower pots on their heads - headline June 28, with hyperarticulate bar band the Hold Steady sharing the bill. The next day, it's Boston Irish punkers the Dropkick Murphys, along with Less Than Jake and Goldfinger. School of Rock All Star bands will play throughout the fest, which starts on a Friday night at the Electric Factory with a Butthole Surfers show. June 27-29 at the Electric Factory and Festival Pier.

Sonny Landreth. The guest list on Louisiana guitarist Sonny Landreth's new album From the Reach, which includes Eric Clapton, Dr. John, Vince Gill, Mark Knopfler and Jimmy Buffett, gives you an idea of the esteem in which Landreth is held by his peers. With good reason: Landreth's slicing and dicing slide-guitar work is peerless, and blends effortlessly into his piquant Cajun rock songs. He'll be a frequent visitor this summer, doing a solo gig at the World Cafe Live on June 13 and performing with Buffett at Citizens Bank Park on June 14, then coming back around with his band to Haddon Heights for the Sundown Music Series on July 9.

Xponential Music Fest. The annual festival presented by radio station WXPN-FM (88.5) again will take place on the Camden waterfront with an impressive combination of national and local acts. It begins with a Thursday night show with the Blind Boys of Alabama and Salvador Santana (Carlos' son), and continues throughout the weekend with Shelby Lynne, Nicole Atkins, Chuck Prophet, Dean & Britta, Amos Lee, Beth Orton, Michael Franti and various other adult-alternative heartthrobs. July 10-13 at Wiggins Park.

Radiohead. With last year's In Rainbows, which was famously made available, initially, via Internet download only on a pay-as-you-wish basis, Radiohead cemented its status as the most celebrated rock band in the world. This summer, the arty and adventurous Englishmen are playing a plethora of festivals across the land, such as Lollapalooza in Chicago and All Points West in Jersey City. Thom Yorke and crew will be headlining their own Camden show, however, coloring in the gorgeously askew soundscapes of In Rainbows at the Susquehanna Bank Center on Aug. 12.

Philadelphia Folk Festival. This year's Philadelphia Folk Festival gets a long overdue infusion of relevance. For the 47-year-old institution, it's a makeover, if not an extreme one. Familiar faces such as Judy Collins, Janis Ian and Cajun dance band BeauSoleil once again will be on hand in Schwenksville. This year, though, Philadelphia freak folk act Espers will curate a stage that includes bluesman Jack Rose and the Baird Sisters, hell-raising sibling act the Felice Brothers, and the husband-and-wife team of Steve Earle and Allison Moorer. Aug. 14 to 17 at the Old Pool Farm.

Stax Soul Revue. Along with Motown and Philadelphia International, the third great soul music operation of the 1960s and 1970s was Stax Records, the Memphis label whose leading African American executive, Al Bell, will be honored at the Rhythm & Blues Foundation awards at the Kimmel Center on Sept. 9. A first-class Stax soul revue comes to the Mann Center for the Performing Arts this summer, with Isaac Hayes, Eddie "Knock on Wood" Floyd, William "You Don't Miss Your Water" Bell, and instrumental greats the Bar-Kays. Aug. 15 at the Mann.

Indoor

Robert Plant and Alison Krauss. The Led Zep front man and bluegrass fiddler's Raising Sand was one of the most seductive surprises of 2007. And clearly, the Golden God is having such a good time exploring American roots music forms that he'd rather hit the road with Krauss than play "Stairway to Heaven" with Jimmy Page every night. Producer T-Bone Burnett, whose new Tooth of Crime was inspired by the Sam Shepard play of the same name, is also on the bill, as is Philadelphia torch singer Sharon Little, who is opening shows on the entire tour. June 8 at the Borgata and July 12 at the Mann.

M.I.A. Last fall's Electric Factory show by the Sri Lankan agit-pop star M.I.A., born Maya Arulpragrasan, was a dazzling, day-glo display of global street sounds and subversive electro-beat pop smarts. Hot on the success of "Paper Planes," the Clash sampling single from her worldly sophomore release, Kala, she returns to town with the Holy F- and Broadzilla DJs in tow. June 5 at the 33d Street Armory.

Tom Petty. This side of George Strait, there's a hardly been a hitmaker who's been more consistently dependable over the last three decades than Tom Petty. And as if to prove the point, the straw-haired rocker, who has scored previous smashes with his longtime band the Heartbreakers and as a solo act, has now delivered yet another well-crafted, instantly catchy CD, this one with his pre-Heartbreakers 1970s band Mudcrutch. Petty will be touring this summer, however, with his trusty old Heartbreakers. June 5 and 6 at the Wachovia Center.

Jenny Scheinman. Violinist Jenny Scheinman has earned herself a reputation as a standout improviser on four instrumental CDs of her own, and in collaboration with the likes of Bill Frisell and Yo-Yo Ma - not to mention gaining work as a sidewoman for the likes of Sean Lennon and Norah Jones. On the eponymous stunner Jenny Scheinman, however, she steps out as a vocalist, working quietly powerful country-folk turf on songs written by Mississippi John Hurt, Lucinda Williams and Tom Waits, and she is equally impressive herself. June 12 at the Tin Angel.

Rickie Lee Jones. Since first emerging as a beret-wearing bohemian with "Chuck E.'s in Love" way back in 1979, Rickie Lee Jones has had a rambling, shambling career that moved from jazz to pop to standards and back again. Last year's Sermon on Exposition Boulevard was another surprise: a musical interpretation on author Lee Cantelon's book The Words, based on the words of Jesus Christ. Starting June 18, Jones is doing a three-consecutive-Wednesdays residency with a playlist that is expected to span her entire career. June 18, June 25 and July 2 at the Painted Bride Art Center.

Seun Kuti. The youngest son of legendary Afro-pop pioneer Fela Anikulapo Kuti is a born showman. But even if Seun (pronounced Shey-oon) had no idea what he was doing, he'd be hard-pressed to fail with Egypt 80, his father's band, watching his back. In the politicized polyrhythmic tradition established by his firebrand father, the Nigerian scion explores many "African Problems" on his self-titled debut, all of which are discussed to accompaniment of an irresistible dance groove. July 5 at the World Cafe Live.

Al Green. The 1970s soul great Al Green sounds reborn on Lay It Down, the luxurious new CD produced by Philadelphians James Poyser and Ahmir "?uestlove" Thompson. When Green is focused, he's the most soulful man alive, and Poyser and Thompson, along with guests John Legend and Corinne Bailey Rae, have assisted him in making new music that can stand alongside of, if not equal, the incandescent music he made in his Hi Records "Let's Stay Together" heyday. July 20 at the House of Blues.

Aimee Mann. Aimee Mann's never had a solo hit to match "Voices Carry," the smash she scored as the lead singer of 'Til Tuesday way back in 1985. But for well over a decade now, she's been mining a gift for melancholy lyricism and melodic invention in carving out a career as one of the most distinctive American singer-songwriters. "Freeway," her new single, suggests her new @#%&*! Smilers will deliver the goods once again. Aug. 4 and 5 at the World Cafe Live.

Herbie Hancock. Herbie Hancock caught the pop music world off guard when he beat out Kanye West and Amy Winehouse for the album of the year trophy at this year's Grammy Awards. But it shouldn't have been that big a surprise: The jazz piano giant has more than earned the lifetime achievement props that mainstream awards shows like the Grammys specialize in, from his days with Miles Davis' quintet in the 1960s to his pop crossover success with "Rockit" in the 1980s. When he plays the Music Box theater at the Borgata - which also will host Aretha Franklin on July 25 - he'll survey his career and focus on The Joni Letters tribute album that got him the Grammy. Aug. 16 at the Borgata.

Nine Inch Nails. Nine Inch Nails one-man-band Trent Reznor has discovered the Internet as a distribution channel for his unending rage. First, he put the four-disc instrumental album Ghosts I-IV, on sale at nin.com for $5, then he gave away the proper new NIN album, The Slip, scot-free. That hard-driving disc will become available on CD in July, however, and Reznor won't be giving tickets away on his summer concert tour. Aug. 29 at the Wachovia Center.

Contact music critic Dan DeLuca at 215-854-5628 or ddeluca@phillynews.com. Read his blog, "In the Mix,"