Defense Grid

Not multiplayer, but a lot of fun!
I've been playing the crap out of this. It is basically an RTS with a flow/flood gameplay aspect, similar to the old, simple water through the pipes games. You set up different types of turrets around the map to defend against different types of aliens that flood in to steal your power cubes. Once they have your cubes they try to exit the map. Your turrets are invulnerable (the aliens don't shoot) so its really just about how effectively you can wipe out the oncoming flood of aliens using the stationary turrets. They also have flying aliens so you have to have anti-air too. On some maps you can use your turrets to create a maze, slowing the alien advance and giving your turrets more time to do God's work.

There's a demo for it so check it out. It's only $10 (800 points) and I've enjoyed it.