Steam Trading beta program has ended and that the service is now available to all Steam users. Steam Trading allows users to trade in-game items with one another. At the moment, the only games to support the new service are Team Fortress 2, Portal 2 and Spiral Knights. Valve notes, however, that it is "working with other developers to incorporate Trading and Inventories into their games, so keep an eye out for future announcements in the coming months."
Perhaps more interesting than the ability to trade in-game items, however, is the ability to trade Steam gifts. Any game that was purchased as a gift (or as an extra copy as part of a bundle) can be traded for items or other Steam gifts. Naturally, there are some restrictions, most notably that any such game must never have been played. Once a Steam gift has been opened, it can no longer be traded.
As part of the service, all Steam users now have an inventory that can be viewed by other users (so long as your profile is public). Check out the full Steam Trading FAQ here.