"Where Quake meets reality" is the motto the developers of Urban Terror have adopted. While many mods these days tend to focus on realistic game play, Urban Terror takes a "fun over realism" approach to the FPS arena by keeping the Q3 speed and physics.
Great game all around. Can take a while to learn jump tricks but lots of fun when you know how to do them, slide around the map while defeating your opponents. Some servers also host dedicated "jump" maps with obstacle course style gameplay.
Many weeks of fun in this.