The current version of Xcode 4.3.3 for Lion is available via the Mac App Store, the current version of Xcode 4.2 for Snow Leopard is available via direct download.
Developer previews of Xcode 4.4 and 4.5 are available via the Xcode page at Apple, too, as well as older versions of Xcode.
Not all of the links above are available for everybody, for some you might have to register at Apple to use them. While Xcode is generally for free, Apple reserves some tools for registered developers.
