How can you never have heard of Valve? You may have heard of their games, Counter-Strike, Team Forteress, Postal or Half-Life! I'm pretty sure you've heard of Valve even if it is just their games. If you've never even heard of their games, I'd highly recommend The Orange Box which contains Half Life 2, Episode 1 and 2 of Half Life 2, Portal and Team Forteress 2. All really good games.
Left 4 dead because Far Cry 2 gets boring and nobody plays it anymore. Far Cry is good but nobody wants to play it because of the gameplay to some people but left 4 dead is so awesome.
I bought both L4D and FC2 together I must say that FC in the first stages of the campaign is rather annoying since there are quite a few mercenaries around trying to kill you for no reason, which kinda kills the game play for me. Also, the guy in the SP i keep getting briefed from talks too fast and is really annoying but i do have a feeling that it is going to get better later on in the campaign. Left 4 Dead on the other is so much fun i couldn't believe it, although it is very short (Each of the 4 campaigns is around 50 minutes long depending on normal difficulty) the campaign has so much epic-ness and replayaility in both MP and Sp since the AI spawning is different each time which mixes it up a bit as well as the multiplayer being very fun indeed. IMO, L4D is a must buy!