S.T.A.L.K.E.R. is now available on Steam. It was one of the most interesting and atmospheric PC games of this year, and you really should play it - if just to have an opinion on it. It's up there for just $30, so give it up to the Ukrainians. Needless to say, I'll be talking more about Stalker later in the year.
In the meantime: what do you want to see from the Stalker expansion pack?