Valve has unveiled a 'Steam Trading' feature that allows players to swap virtual items, games and other gifts.
"Team Fortress 2 items and Steam Gifts can currently be traded in the Steam Trading Beta," it says.
"Any game you've purchased from the store as a gift, or received as an Extra Copy, can be traded to other users. They can be used to trade for other Gifts, or for items in Team Fortress 2.
"We've added a new checkout option to the Store when purchasing a gift so you can save it for trading or sending later, to support users who want to save games for trade fodder."
Valve says it's working with other developers to incorporate the new feature into more games "in the next few months" and expects Portal 2 should be updated with the feature "reasonably soon".
You can get in on the beta by clicking through to the Settings category in Steam and then opting into Steam Trading Beta.