Planetside 2, still playing BF1942 occasionally, suprisingly (for a 2002 game) there are 2/3 64 player servers full most of the time. As for next may, well, there's the new DooM coming up...
If you like zombie games and own one of the source games (HL2 and such) on steam, try out the mod Zombie Panic Source. It's free.