Blastup 2 is a simple and free hit-the-other-guy-with-a-grenade-or-something-in-the-head game, featuring 6 weapons/tools, a lot of options and 13 landscape themes.
You can play against a computer player, the guy/girl next to you or someone else over the network/internet.
The requirements to run the game is no more then a P2 with MMX running Windows.
For more info, take a look at the features page.