I am currently running two games (Both the WLD) and playing in the weekly game.
For the two games I am running we have the following books "in play" The PHB, all 4 complete books and I generally use just the MM but have occasionally thrown in a monster from MM3.
The weekly game we have the PHB, the complete books, the planar handbook, and the DM is using MM, MM2, MM3, Fiend Folio, the Draconomicon, and other various suppliments he gets his hands on. Oh, and were using the UA action points system execept that we get 6 points every level and they reset when you level (so use them or lose them)