Steam is currently testing a new system allowing players to securely and legitimately trade in-game items for other virtual goods - including full games.
According to the Team Fortress 2 blog, Steam Trading is currently in opt-in beta testing and limited to Team Fortress 2 items only.
Those who choose to participate can take in-game items like Team Fortress 2's famous hats - which can be purchased, won in special events and promotions, found in random drops, or crafted - and offer them for trade.
In addition to or in exchange for items, players can trade games not yet installed to their Steam libraries.
Presumably this mean you could gather a number of rare or desirable hats for free, and trade them in for a game without paying a cent - and get rid of extra copies received as gifts or in bundles.
After opting in to the beta, trading can be initiated through Group Chat, or with other testers on your friends list through a new menu item. A tutorial will be triggered on first attempt.
Team Fortress 2 had previously implemented a trading system which Valve has significantly overhauled before opening it up to other virtual goods.
Steam recently opened its catalogue to freemium games and micro-transactions, which provides an interesting context for this new system.