Steam Trading Beta lets you 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).
Want to try it out? In Steam, go to Steam > Settings, and opt into Steam Trading Beta. That's it! Now your Steam Community profile will have an item inventory, and you'll be able to view the item inventories of every public profile in the Steam community.
It seems like a good way to finally get rid of the 20 million free copies of Portal I've amassed, and while currently the system only supports TF2 and Steam gifts, Valve are working on introducing Portal 2 and a number of third party games in the coming months. This also seems like a definite hint that upcoming Valve games will have a similar system to TF2, as it would be a lot of work for little gain otherwise. You can read more on joining the beta on the official Team Fortress blog.
