ConnecTank

For the most fun, you have to play this with at least two players. Three or four is even better. But you can have fun in single player as well. If you've ever played Rocket Slime for Nintendo DS, they basically took the tower battles from that game and expanded on the gameplay (but took away the whole RPG stuff). You do get some stuff like ammo upgrades, new tanks to unlock and a map, where you choose the way to the boss. But ultimately, I wish there was a litte more than just the battles.

There's 39 achievements to unlock.