I greatly enjoy the simplicity and action when playing BZFlag (http://bzflag.org/). It's multi-player, free, and fast. The graphics are out dated, but I think that's part of the charm. And anyone can make maps.
I have two questions...
First, does a variation of the game exist where you can trade in kills for upgrades, instead of just being able to use one flag at a time? Think of the Sturgeon variants of NetTrek.
Is there an air combat equivalent of BZFlag? The grphics don't have to be good, but the idea of 20 to 200 person dog fights sounds appealing at the moment.