Valve had added a Steam Trading Beta feature into their popular online service, that will allow the trade of in-game items and even Steam gifts.
At the moment the only trading available is for Team Fortress 2 items, but the idea is that you'll be able to trade items for items, gifts for gifts, and then a mixture of the two. Valve are also hoping to get Portal 2 into the beta and are also coaxing third-party publishers into the mix to flesh the service out.
Not a bad idea for active Steam users. More info can be found on Steam's site right here