Urban Terror is a mod turned standalone thats on the Quake 3 engine. It features realistic gameplay, much like in Counter Strike, except for a twist: you can jump off of walls and do other crazy things like that. Also, all the weapons are available at spawn, instead of a buy system. The same applies to the equipment. Another interesting idea is instead of realistic looking skins, there are orange and blue teams.
The gameplay in Urban Terror is absolutely fantastic, borderline perfect. The weapons connect when theyre supposed to connect, and not at any other time. Movement is easy to get used to.
System Requirements:
64 MB RAM
8 MB VRAM
Around 650MB disk space
Obviously the higher your specs the smoother the gameplay.