Steam is offering Team fortress2 for this weekend. Yes, steam client must be used for to install it. Here is the link.
http://cdn.store.steampowered.com/message/19285217737055161/?l=english&cc=US
I have this already, but havent got a chance to try it yet so I am unsure about how good it is. It does carry a 92 score on steam which is pretty good.