Urban Terror is a free multiplayer first person shooter. Unlike old versions, it no longer requires Quake 3; anyone can download it and play for free. Urban Terror is available for windows, linux and macintosh, the current version is 4.1.
Urban Terror can best be described as a Hollywood tactical shooter; it is realism based to a certain extent (environments/weapons/player models), but also goes by the motto "fun over realism" (fast gameplay and lots of action). This combination of reality and action results in a very unique, enjoyable and addictive game.

Installation
To start with, download the game from their site: Urban Terror
Once you have the game downloaded, extract the zip file.
In the folder UrbanTerror there is a file called: ioUrbanTerror.i386 (if you are using 64bit foresight, it's ioUrbanTerror.x86_64)
First you need to make it executable: right click on the above file, choose 'Properties', then choose 'Permissions' tab, check the 'Allow executing file as program' option.
Then you only need to double click on the ioUrbanTerror.i386 file to start the game
Or install it with terminal:
sudo conary update UrbanTerror{,-maps}=zinden.rpath.org@fl:2-devel
Possible problem
You get a black screen only, then you need to do this:
Start Nautilus, go to home folder.
Go to menu: Show, Show hidden files
Go to folder: .q3a
Go to folder: q3ut4
Open the file: q3config.cfg
search for this: seta r_mode "4"
change to: seta r_mode "8"
Save and try to start the game again.