Clonk
German puzzle/action/strategy game. Kind of like a mix of worms and lemmings. In it you have control of some "Clonks", you harvest materials, build stuff, and possibly fight depending on the scenario. All of their games, except for the latest (Clonk Endevour) are freeware. Clonk Planet has multiplayer, which is a garenteed addition to my next LAN party this weekend.
Multi has both coop and competetive scenarios. All games are open-source I think.