Cedega
Cedega is a commercial version of wine that is geared towards games. It was previously known as WineX, but changed its name with version 4.0 to Cedega. It features a very large database of games, and forums. It comes with excellent support for paying customers, and good support for non-game applications.
It is produced by Transgaming Technologies and forked from wine years ago. It currently uses the Aladdin Source License making porting code between Cedega and wine very hard (wine is under a more liberal license known as LGPL).
Transgaming does provide support and binary releases to paying customers. Otherwise as with wine what you get is the current CVS.
See the wine page for further information, or Gaming via Wine or WineX for information on playing games under Cedega.