So uh, I'll give you a hat if you give me a game :>

[Danny]

A few days ago, Valve started a beta of something they're calling Steam Trading. If you participate in it, you get the possibility to trade, say, a TF2 hat for Left4Dead - for example.

Only a few select Valve games are part of the system at the moment, but they're working with other game devs to include item inventories, trading and such.

More:

http://www.teamfortress.com/post.php?id=6040