Steam Item Trading Feature In Beta

According to a post on the Teamfortress 2 blog, you'll soon be able to trade in game items for any other in game items from any participating game on Steam(you can expect those other free to play games to offer this soon). This also applies for games you have not yet installed on your Steam library(in that case, you can change it for something else). This feature is currently in beta(which you can opt in for) and only applies to Team Fortress 2 in game items and you can't trade those for other games though(although they say you will be able to do so when the feature is gone live officially. You can read the official Trading FAQ here

Source: Team Fortress 2 Blog