Last month Steam launched a beta for a trading feature that allowed Steam users to trade gifts and in-game items to other Steam members from Team Fortress 2, Portal 2 and Sega Adventure Spiral Knights. This feature has now gone live.
An auto-update kicked in today that made the feature available to all Steam users.
Valve is currently working with other developers to incorporate this feature into their games. The full Steam FAQ can be read here.
Steam Trading Feature Goes Live [Eurogamer]