Just the other day, I was pondering why I hadn’t tinkered with Steam Trading much yet. It allows you to swap full games (if they are Steam gifts) and in-game items with other players. As of today, the feature is no longer in beta.
Valve has included support for Portal 2 and Spiral Knights, on top of Team Fortress 2. Rather than having to go into settings to enable the functionality like before, you can simply see your personal inventory on your Steam community profile. Once they get the ball rolling and more developers sign up, this could be very useful indeed.