Apple's new iCloud service will let users access their digital content from any device, but other companies (like Google and Amazon) already offer similar services.

Apple CEO Steve Jobs re-emerged from his latest medical leave June 6 to show off the company’s latest innovations, which include a new cloud-based storage service that could make it easier to teach with digital materials downloaded from the iTunes or iBooks libraries.

Jobs was onstage for less than 30 minutes during a nearly two-hour event that primarily featured his subordinates. Ever the showman, he announced that the company had struck licensing agreements with all the major recording labels on a new music synching system.

It will allow people to put all the songs they have ever bought from the company’s iTunes store on up to 10 devices at no additional charge. Apple is offering to do the same thing with all books and applications previously purchased through its online stores.

All future iTunes purchases will be sent to all the devices automatically, too. None of the transfers will require devices to be plugged into a single computer. This will happen automatically over wireless connections.

“Keeping all those devices in sync is driving us crazy,” Jobs said.

Jobs’ keynote address at a conference for application developers marked his first onstage appearance since he unveiled the second version of Apple’s tablet computer, the iPad, three months ago.