The collection and sale of virtual items (read: hats and gifts) is a key part of Steam's Team Fortress 2 monetization scheme, and Valve has announced that the Steam Trading Beta is now live. This service lets you easily trade Team Fortress 2 character items and Steam Gifts from within the client, making for a much more streamlined approach to bartering than constantly spamming the in-game chat with requests.
An inventory of each player's items and gifts (games that you own but haven't yet activated on Steam) will be visible on their profile depending on privacy settings, and trades can be organised through group chat or via your friends list. Portal 2 and "several third-party games" will be included over the coming months. Here's an FAQ explaining the whole thing.
You can try it out by going into Steam settings and clicking into the beta. Let us know what you make of it.