I think Enemy Territory is still going to be your best bet. There just are not many free FPS games with better graphics or gameplay then ET. I believe that Tribes 2 was released for free a while back, it definately has dated graphics, but it's a fun lan party game. And as JBox mentioned, Quake shows up at pretty much every lan party for a reason.
For strategy games, my current favorite is Supreme Commander, but you'll probably have to buy seperate copies to run it.