Urban Terror

Urban Terror started as a realism based "total conversion" mod for Quake III Arena. Nowadays, Urban Terror is played using the free, stand-alone, ioquake3 engine modified, which doesn't require Quake III Arena, in addition to being an add-on to id Software's Quake III Arena.

Minimum system requirements: 

CPU: 1.2 GHz (x86 compatible)
RAM: 256 MB
GPU: OpenGL-compatible with 128 MB VRAM
HDD: ~5.0 GB
AND: Quake III Arena >= 1.32 (If you want PunkBuster support)

External links:
Developer: FrozenSand