I was about to write bestdigital distributor, but I remembered GOG.com and relegated Steam to biggest. It certainly is one of the better modern game digital distributors, of course, what with Valve games being there and nowhere else.
So, getting back to the topic, Steam has uncorked the finely beta-aged wine of Steam Trading and allowed all Steam users access to its digital trading feature. Steam Trading is, very simply, a digital trading service built atop the Steam client that allows users to trade Team Fortress 2, Spiral Knights and Steam Gifts with one another. Valve have also said that trading of items from Portal 2 will soon be open. Steam Gifts can be traded for other games or TF2 and Spiral Knights items as long as you have not opened the gift:-
Any game you’ve purchased from the store as a gift, or received as an Extra Copy, can be traded to other users. They can be used to trade for other Gifts, or for items in Team Fortress 2 or Spiral Knights
This is quite a lovely step up by Steam and is a boon for many online game trading forums that relied on trust instead of a portal like this for their trading. Of course, you could also stock up a sizable collection of hats from Team Fortress 2 , but that is completely up to you.