Valve has announced Steam Trading, which is currently in beta. It allows you to “securely trade your in-game items for other in-game items from participating games. You can also trade Steam gifts (games on Steam that you’ve purchased but not yet added to your Games Library).”
To opt in, go to Steam > Settings > Account, and you’ll see a section on beta participation. You can view your own item inventory — and other people’s, if their accounts are public — by way of your Steam profile. There are no regional restrictions for trading, which is cheerful news for gifts.
As for expanding what can be traded, “Portal 2 should be reasonably soon and we hope to have several third-party games in the next few months.” There’s an FAQ over here.