Cocos2D is an open source framework that developers can use, along with Apple's Xcode software, to help build 2D iPhone games and apps for the App Store. A lot of popular titles are built with this framework, so many that I can guarantee you that you've played some of them.
CocosBuilder is another tool for developers. It is a graphical interface for setting up all of the various sprites and widgets used in Cocos2D. And not only has CocosBuilder just released version 1.1, but it's now been made open source, which means anyone can download and use it to build scenes for Cocos2D.
The app's creator, Viktor Lidholt, says he's been hired full time by Zynga to build and grow CocosBuilder, so we expect even more updates. Unfortunately, this isn't the kind of thing you can still just jump in on. While Cocos2D makes things relatively simple for developers, it requires a solid working knowledge of both Apple's Xcode software and the Objective-C language. It's not quite as easy as a graphical SDK like Gamesalad, and CocosBuilder is still in beta, with all of the issues that come along with that.
But it's still a great tool for devs using Cocos2D, and the fact that it's now officially open source should be a nice boon for those interested. CocosBuilder can be downloaded directly from the site.