You can now trade in virtual items for other virtual items through Steam.
Steam Trading Beta lets you do that, and trade Steam gifts: games on Steam you've bought but not yet added to your Games Library.
Currently Team Fortress 2 items and Steam Gifts can be traded.
You can try it out now by going to Steam, then Settings, then opting into Steam Trading Beta.
"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," Valve said.
You can invite people to trade through Group Chat, and trade with anyone in your Friends List who's also opted in.
To do so, you need to right click on their name and select Invite to Trade. Once the offer is accepted, a trade window will open up.
There's more on the Trading Beta FAQ.