http://steamcommunity.com/market/

For those of you who don't know, the Steam marketplace just recently launched into beta. It's going to be a marketplace in which you can use your steam wallet to buy or sell various items with other players, kind of like an auction house in an mmo.

Right now it only supports 2 of Valve's games, Dota 2 and Team Fortress 2, with limited things that can be sold. Later on more items like guns, hats, dota 2 hero items, and more will be available to be exchanged, as well as games by other developers.

I've made a fair amount of money selling useless mann co supply crates from tf2 for 1 cent each, and from selling some event items from dota 2 for 20-50 cents each that only take like 20 minutes to obtain just regularly playing games. The money you make selling things gets added to your steam wallet, so you can buy all kinds of games just earning money playing others

also, I have some Dota 2 beta keys if anyone is interested. ^_^