There is an interesting game which is called Counter-Strike.

 

There are two teams:Terrorists and Counter-Terrorists. They are fighting on the map.

 

There are some types of maps: as ( terrorist have to kill the VIP, counter-terrorists have to protect him ), cs ( terrorists have to protect houstages, counter-terrorists have to rescue them ), de ( terrorists have to plant a bomb, counter-terrorists have to defuse it ) fy ( deathmatch, all weapons are lying on the respawn ) and many others.

Each round teams have to buy weapons first. Then each team is going to do its task.
People can play this online or with computer opponents. It's free.
There are a lot of versions of this game. I think version 1.6 is the best. The game also has a lot of patches. I am playing on 36.

     
 
 
Wikipedia info:

 

                                    

Counter-Strike (also known as Half-Life: Counter-Strike, Counter-Strike 1.6, or simply CS)

is a tactical first-person shooter video game developed by Valve Corporation which originated

from a Half-Life modification by Minh "Gooseman" Le and Jess "Cliffe" Cliffe.

 

The game has been expanded into a series since its original release, which currently includes Counter-Strike:

Condition Zero, Counter-Strike: Source, Counter-Strike: Anthology and Counter-Strike on Xbox.

 

Counter-Strike pits a team of counter-terrorists against a team of terrorists in a series of rounds.

Each round is won by either completing the mission objective or eliminating the opposing force.

The game is currently the most played Half-Life modification in terms of players, according to GameSpy.
The date of realise of version 1.6 is September 15, 2003.