SAN FRANCISCO - Apple CEO Steve Jobs briefly emerged from a medical leave yesterday to unveil a free service that lets customers share calendar entries, songs and other files among their devices more easily.

The company also announced software to make Mac computers behave more like mobile devices and Apple's mobile devices more like rival smartphones.

Jobs received a standing ovation as he appeared at Apple Inc.'s annual developers' conference, his second major public appearance since he went on medical leave in January for unspecified reasons and duration.

Apple's stock fell $4.05, or 1.2 percent, to $339.33 in afternoon trading. The stock increased in the morning, but fell soon after Jobs left the stage.

An iCloud account will store user information from several devices, including iPhones and iPads, and make sure the same contacts, calendar events and files are available on all of them.

ICloud, free for now, will also allow customers to store their music online. Buy a song on iTunes once, and it will be available on up to 10 devices.