Apple has posted the videos and slides from the technical sessions of this year's Worldwide Developers Conference yesterday. The videos are available for free, but you need to be a registered Apple developer to watch them. (You can register at https://developer.apple.com/.)
As in previous years, there's a lot of good stuff here for iOS and OS X developers, as well as some sessions on other parts of the Apple ecosystem like Safari and iAds. Also featured this year are several sessions related to authoring content for the iBookstore using both EPUB 3 and Apple's proprietary iBooks Author formats. There are also a good number of sessions focused on getting the most out of Xcode and the Apple toolchain.
I've watched many of the videos from previous years and they are quite useful. If you are an experienced iOS developer, these sessions are a great way to help you get up to speed on the latest stuff. If you are new to iOS, the WWDC sessions, especially from previous years, contain some great overviews that will help you get up to speed fast. I highly recommend them.