Trem is a free, open-source game, based on ioquake3.
It's quite similar to the quake 2 mod 'Gloom'.
Anyho, it's as fun as slagging on bogans - so download it and give it a spin:
http://www.tremulous.net/maps and mods at
http://www.tremulous.info/-TSB out[SIZE="1"]looking forward to pwning youse nubheads[/SIZE]