Originally a Quake 3 Mod, Urban Terror now works as a standalone online multiplayer first person shooter. Forget boring realism, this is all about fun (and large scale killing).
It's best to start with the tutorial, a short video that takes you through the controls. They are fairly standard, but there are nice abilities like wall jumps, strafe-jumping and crouching power-slides. The graphics are obviously a bit dated, but the advantage of that is the game will run perfectly smoothly on all but the oldest systems.
Finding a game is still pretty easy, you just choose from the list of hosted games and join one. Most seem to play either team deathmatch, which is popular as it's just a timed free for all, with unlimited lives, or team survival, where each player only has one life and the objective is to take out the opposing team. They are good examples of how rules change the game - deathmatch is fast and furious, while survival is a bit more tense. The exaggerated physics make for amusing games, and even as a rookie you'll soon be gleefully taking out other players with ease.
There's a good choice of arms, allowing you to play in the style you want, from up close and personal with automatic rifles to sneaky sniping or grenade launching brute force. If there's a negative in all of this it's that there are always times when you'll feel you've been unfairly killed - usually by a skilled sniper - but you'll be back in the action before you feel any major resentment.
Despite its aging engine, Urban Terror is still a great multiplayer shooter and you'll be hard pressed to find more online over the top fun for free.