WineX is a fork (see everyone's writeup) of wine being developed by TransGaming and licensed under the AFPL. They're intending to release it back under the Wine license: see the TransGaming node for details.
Specific things which have been improved in the WineX tree are mainly DCOM (mainly so that InstallShield works), code necessary for copy protection to work (this code is not in CVS at present due to legal issues) and DirectX.
Their Direct3D implementation relies on translating the function calls into their equivalent OpenGL calls, and they've implemented most of the differing versions, including the latest DirectX 8.
It manages to achieve compatibility with a surprisingly large set of Windows games, and at a decent speed, sometimes even faster than they run under Windows.
They occasionally merge non-core changes they've made back into the wine tree under the Wine license, but this doesn't happen often, as it is a lot of work for them to do this often, and there is not really a commercial reason for them to do it.
You can download the source code from their CVS repository on SourceForge, or packaged binaries from the TransGaming web site if you're a subscriber.