Epic Games started development on a new Unreal Tournament in collaboration with the community and Unreal Engine 4 developers this week, and that makes this a decent enough time to discount the existing games in the franchise, I'd say. (Though you surely own 'em already, right?)
There's exactly such a deal running on Steam until Wednesday, May 14. Unreal, Unreal 2, Unreal Tournament: Game of the Year Edition, Unreal Tournament 2004: Editor's Choice Edition, and Unreal Tournament 3 Black are half price. You'd do well to play UT2K4.