"Today, we're going to talk about software," he said, before handing off to Phil Schiller, Apple's senior vice president for worldwide marketing.
Jobs returned to the stage to introduce iCloud, a service he said would make it possible for users to access photos, music, documents and other content at any time from any device.
He demonstrated how photos taken with an iPhone were visible on a user's iPad moments later.
Immediate reaction to the new offerings was positive, but many in the crowd of 5,200 were still reacting to Jobs himself, who wore a trademark black mock turtleneck and blue jeans.
The June event was his second public appearance since his most recent medical leave. The other, in March, was for the release of the company's successful iPad 2 tablet.
Jobs concluded his resignation letter with a look toward Apple's future with Cook as CEO.
"I believe Apple's brightest and most innovative days are ahead of it. And I look forward to watching and contributing to its success in a new role," he wrote. "I have made some of the best friends of my life at Apple, and I thank you all for the many years of being able to work alongside you."
ABC News' Neal Karlinsky contributed to this report.