Hm. Isn't Freespace 2 "technically" free? I'm not too clear upon whether or not the source was released to the public domain, but... I still love it. I want Freespace 3.
Pocket Tanks is a decent 1v1 type of game; tons of weapons, 2D, tank vs tank, adjust angle/velocity that type of thing. The basic game has maybe ~30 weapons? Not sure, but I think it should also have LAN (at least the full version has).