Death matches, capture the flag, bots and Mods are just some of the gaming innovations that have came as a result of the multiplayer shooter genre.

Team Fortress 2 - Best Online Shooter of 2007 
One of the best team shooters ever. Sports cartoon-style graphics and loads of personality.

WarRock
Free online shooter with the option to buy more high-powered weapons.

Crysis
One of the best looking games ever so not for all PCs. Focused on single player, but does have a multiplayer mode.

FEAR Combat
Free multiplayer version of FEAR.

Soldier Front
Freebie that features distinctive weapons from task force characters. 

Unreal Tournament 3
Latest installment of one of the best shooter franchises boasts a new Warfare mode and high-tech vehicles.

Tremulous
Free open source game that combines FPS elements with RTS elements. 

Max Payne Kung-Fu Mod
Dish out the pain kung-fu style in this innovative mod that even gives Max a pony tail.

Battlefield 2  
Not only outdoes the original and BattleField 1942 and Battlefield Vietnam but has to rank as one of the best shooters yet. 

Doom 3
When you think of Doom 3, think power and fear! D3 does the vaunted franchise proud with amazingly intense game play and simply awesome graphics. You won't be armed to the teeth with weapons where you can mow down hapless monsters. Instead, you will need to tough it out corridor-by-corridor in order to survive. Needless to say, you will need an extremely powerful PC with a state-of-the art graphics card to fully experience this new classic.   

Control Monger
Free online FPS where bots go to war! 

FarCry
Forget dungy dungeons and drab planets, this beautiful shooter takes place in the sunny tropics. 

S.T.A.L.K.E.R
Highly-anticipated shooter that should be brilliant. 

Half-Life 2
The long-awaited sequel to one of the best games ever. Will cater to the mod community even more this time. 

Quake 4
Went back to a story format after Quake 3 got squashed by the Unreal Tournament franchise. Did not make the same impact as Half-Life 2 or Doom 3.

Tribes Series
One of the best multiplayer shooter franchises ever. Believe it or not, Tribes is actually a spin-off from the failed Sierra game, Starsiege. You can now download Tribes 1 and 2 for free or you can wait on the latest installment in the series, Tribes: Vengeance. 

Unreal II
Although a multiplayer version did not originally ship with the game, a multiplayer version was thankfully added for free. 

Battlefield Vietnam
Takes most of the great elements of BF 1942, cranks up the rock and roll, puts it in the jungles of Vietnam.  

PlanetSide 
First person shooter MMORPG that features vehicles, squads, and hacking. The current leader of the emerging MMOFPS genre. 

Vietcong
Take on your friends in explosive jungle warfare!

Ghost Recon
One of the most strategic online shooters that you will find and one of Tom Clancy's best games ever.

Unreal Tournament Cheats and Strategies Blow away the competition with this wide assortment of cheats and strategies for one of the games that revolutionized the online shooter genre. America's Army  Download the full version of one of the best multiplayer shooters available. Half-Life The undisputed king of mods in the FPS genre or any genre for that matter. No game has near the number of mods as Half-Life has. This has contributed to Half-Life being recognized as one of the best games ever.

Battlefield 1942
The game that revolutionized multiplayer shooters with a WWII theme, vehicles, and great player interaction.

Delta Force and Joint Operations
Tactical shooters with their own free online service called NovaWorld.

XIII
Cel-shaded FPS where you can fight other players in levels made by you.