ok. i love TF2 so i'm gonna hop on that
Looks like TF2 is still Windows only?
Portal is available for free. Civ IV series is available, plus a ton of indie games. Not much yet but i'm sure it will grow.
Gonna install it on my wifes Mac tonight so she can play some of the games. It looks like any existing games you own labeled with the "Steam Play" icon means you do not need to purchase a separate copy for each platform, which is nice.