Valve is trying out something new with the launch of the Steam Trading beta for its popular digital delivery service. Participating users can trade select in-game items and full games with one another as long as the trade is approved by both parties.
The beta is open to all Steam users and easy enough to access; simply open the desktop client and go into the Settings menu. The Account tab contains a section for "Beta Participation," which you can access to get yourself squared for Steam Trading.
Once you're signed up, head to the client's Community section and click "View Inventory" on the right side of the page, under the Actions tab. Team Fortress 2 is the only game that allows for the trading of in-game items at this point, but it appears you can also trade any "Gift" game you've received. Expect more titles to be added too as the beta progresses.