Sauerbraten, its free and open source. It supports multiple gamemodes (standard deathmatch, instagib, many team modes and capture mode which is like BF2) and it has pretty good graphics but still runs on older systems (like systems with GeForce 2s). It also runs on Windows, Linux and Mac OS X, all compatible with eachother online.
https://sourceforge.net/projects/sauerbraten/files/ - Download page
http://sauerbraten.org/ - Sauerbraten Homepage
http://cubeengine.com/forum.php4 - Official Sauerbraten Forums
http://forums.xisto.com/no_longer_exists/ - Unoffical Newbie Guide for Sauerbraten
Check it out!