Valve's in-game TF2 item store is about to become an out-of-game item store. They're trialling something called Steam Trading, which primarily involves swapping your TF2 unlocks (i.e. those damnable hats, mostly) for other games.
It's an old-fashioned barter system in new-fangled clothes. What happens is you invite someone on your Steam friends list or who you're in a group chat with to trade, and can offer up your various TF2 items to the other guy. In return, he or she can offer you other TF2 items - or to gift a game to you. You can't do this with any old game in your Steam library - only games you’ve purchased from the store as a gift, or received as an Extra Copy.
To support all this, your Steam Community page will include a new inventory screen - similar to that in TF2, but available out of game and, eventually, for multiple games. There's also a new option to store games for trading when you buy them, instead of activating them on the spot.
So, short-term, this potentially solves the problem of what to do with your various spare copies of older titles from promotions or pre-orders. You're able to trade gift-designated games for other gift-designated games, by the way, so it could be a way to stock up your games library without spending too much.
Long-term, it could be a way to get hold of rare in-game items possibly for a knock-down price (if you're canny about buying games during sales and whatnot), and without giving money directly to a stranger.
Not sure how I feel about it yet. I suspect it's going to most come into its own when more games get added to the pan-Steam item trading - so, for instance, trading a TF2 hat for a Portal 2 hat. Says Valve, "Portal 2 should be [added] reasonably soon and we hope to have several third-party games in the next few months." Which is a fascinating prospect - and further increasing Steam's status as almost a platform unto itself rather than just a launcher/store. It almost makes it a metagame, even.
It's an appealing idea in principle - offloading crap I don't want for stuff in another game that I do, though that rather depends on certain items not being perceived as overly common and valueless. And the idea of so many people in so many games being even more fixated than they already are on collecting frippery is a mite fearsome and in-game chatter trying to organise trades could be hugely disruptive. At the same time, it seems like a natural next-step for Valve's hat-centric masterplan.
Steam Trading is currently being trialled in beta, which you can opt into via Steam's settings right now. More details about how it works and what's planned for later are in this official FAQ.