Tetris!, Umod format.
Tetris! is a mod that works as an extension to the UT GUI. It's a simple tetris clone, with a few interesting features, such as themes and some secrets. A small, but fun modification.
Once you have successfully installed Tetris!, run Unreal Tournament. From the Mod pulldown menu in the UT GUI, select Tetris!.
From here, you are on your own. The arrow keys are the default keys; these are configurable in Options.
There's isn't much to explain. I guess everyone in earth have played Tetris... :P