There is a new beta program running at the moment for your steam client which allows you to trade in-game-items, like hats in Team Fortress 2, with your friends directly via the steam client - rather than in game. Worth checking out as you can also trade other things like games for in game items as well. This opens the door to a wide range of posibilities for game developers - most of which will surely involve you parting with more ca$h.
To unlock the beta client go to:
- 'Steam' menu item.
- Account tab.
- Beta Participation heading and apply to the beta program of your choice and then restart the client.