The Steam trading system is officially out of beta. The new system will now allow all Steam users to openly trade in-game items from TF2, Portal 2, and the free to play MMO Spiral Knights for other items (from any of the games), as well as for unredeemed Steam games. The beta was apparently a huge success, with over a million items traded during the testing period.
To celebrate the release, select items are on sale in the TF2, Portal 2, and Spiral Knights in-game stores. You can check out the feature yourself by picking a friend from your friends list, opening up a chat window, and and using the drop down menu next to their name to select "invite to trade."
You can also find out more at Valve's Steam Trading FAQ.