Apple has released Xcode 4 for free for all registered developers. The new Xcode offers a streamlined interface "that integrates UI design, coding, testing, and debugging, all within a single window." In addition, the embedded Apple LLVM compiler underlines coding mistakes, much like Microsoft Word underlines spelling mistakes, as you type and can even correct your mistakes automatically.
Interestingly, Apple has also decided to sell Xcode 4 to anyone for only US$4.99 through the Mac App Store. The reasons that Apple has decided to sell Xcode 4 on the Mac App Store are not clear. It's possible, however, that the company wants to spur app creation for both the Mac and iOS App Stores for developers who normally wouldn't have access to Xcode (or indeed, even know of its existence).