One of the drawbacks of purchasing digital games, whether it be through Steam, Xbox Live Arcade, or most anywhere else, is the inability to trade or sell those games later. This is less of an issue when it comes to PC games, as CD keys and online registrations have made it more and more difficult to hand off a computer game to someone else.
Enter Steam Trading. It’s not quite the perfect solution to these problems, but it is one step in the right direction. By opting into the new beta, you can trade both in-game items and certain games with other users who are also enrolled in the beta. Game trading is limited to gift copies that you’ve yet to send to someone else or add to your own account. Only Team Fortress 2 items can currently be traded, though Portal 2 should be in “reasonably soon” followed by “several third-party games in the next few months.”
You can get into the beta by going into your Steam settings and opting in on the Stream Trading Beta. From there, you can initiate trades from group chats or by right-clicking on a friend’s name. That brings up the trade window, which will instruct you on how to use it the first time it’s brought up. There’s also an FAQ here.