Valve have announced that its Steam Trading feature is now live and is available to everyone.
Steam trading lets players trade items from certain games as well as Steam gifts, games on Steam that you've purchased but not yet added to your Games Library and Extra copies of games.
Currently only 3 games support Steam Trading: Team Fortress 2, Spiral Knights and Portal 2. You can find new items for sale within these games' in-game stores.
There are a load of questions on the Steam Trading FAQ page.