GoGamer has Doom 3 for 29.90 + s+h in their madness countdown this week. Plus, you can get a free slinky.
Linky
If you're looking for anything else, they have unreal gold for $2.90, Unreal Mission Pack for $.99, Descent
Freespace and Exp. for $9.90, etc.
Up to 3 games for same s+h price.