Steam members can now trade items from Spiral Knights, and Team Fortress 2 as well as gifts purchased (but unopened). Although these are the only games supported for item trading, Valve has said that "Portal 2 should be reasonably soon and we hope to have several third-party games in the next few months." With that being the case, hopefully we will see more options to optimize this item trading network.
Another new feature is the Steam Community Inventory. For now this inventory is view-able by you while Valve works on the privacy settings/features. Eventually you will be able to see every public profile's inventory, and send messages to those players to initiate trades, but you will also be able to keep your profile private and only available to your friends if you prefer. For all the details on the inventory and actual Steam Trading system, check out Valve's FAQ.
With Millions of Team Fortress 2 hats traded successfully during the open beta, it's no wonder they rolled this out so early. To see the official announcement head over to Steam's News page.