MegaMek
PC game remake of the old tabletop wargame, Battletech. Free/Open source and has hotseat, network, and AI for solo. Runs on Java, so its platform independent.
Looks good, especially because I'm a huge nerd and I love tabletop RPGs and wargames in any form. 4 or 5 meg download, haven't finished it yet so I can't comment on quality.