Rocketmen: Axis of Evil - It's Smash TV meets Flash Gordon. 4 Player Co-Op
WOTB: Commando 3 - Smash TV style game. 3 player Co-Op.
Assault Heroes 1 & 2 - Jackal meets Smash and it's 2 player Co-Op and one of my personal favs
Worms - Duh.
Undertow - Underwater conquest type of thing. multiplayer but can be a bit picky with firewalls. Seriously, it gives you a message about firewall crap.
I will add more at later dates. But these titles are definitely worth snagging.