Team Fortress 2 Beta is available to play on steam if you pre-purchase. I must say though it looks pretty much finished though. Great fun game. No grenades in this one which I think make the teamwork more important.
For those who prefer the more TFC style of gameplay complete with conc jumping etc check out http://www.fortress-forever.com/ I will be playing both of these games :)
On another note I am able to transfer a copy of Half-Life 2 and Episode one to somebody elses steam account as I already have them. If anyone is interested let me know. I will be biased towards people who contribute alot to the site though :) .