Team Fortress 2 Blog announces Valve is beta testing a new function to allow the trade of in-game items and gifted games between players. A Steam Trading Beta FAQ attempts to answer all your questions about this, and here's a bit on how it works:

Steam Trading Beta lets you securely trade your in-game items for other in-game items from participating games. You can also trade Steam gifts (games on Steam that you've purchased but not yet added to your Games Library).

Want to try it out? In Steam, go to Steam > Settings, and opt into Steam Trading Beta. That's it! Now your Steam Community profile will have an item inventory, and you'll be able to view the item inventories of every public profile in the Steam community.

Now that you've opted into the Trading Beta, you can invite people to trade through Group Chat, and trade with anybody in your Friends List who's also opted in. Just right-click on their name and select "Invite to Trade" from the dropdown menu. Once your friend accepts your offer to trade, a trade window will open up. The first time you open the trade window, a step-by-step walkthrough will guide you through your first trade.