When they're not spinning around and around in their expensive chairs, weaving hats, or thinking up other ways to not make games that people want ("Shall we all got to Hawaii this week, or shall we crack open that HL3 design document?" "Aloha! Aloha! Aloha!"), Valve's brain drones are at least attempting to create a community of people that can earn a living from making and contributing to games. Sometimes it's a bit broken, like Greenlight is right now. Other times it can be so successful that Valve can afford to share the wealth between organisations that contributed to the success, but had no way to monetise their involvement. So now, when a community item in Team Fortress 2 or Dota 2 is sold in either game's store, it's possible for some of Valve's take to be directed to the likes of Blender and Polycount. Ooh, just thought up a new word for it: Valvetruism.
It's all in the item maker's hands. When they submit to the market, they'll be presented with a list of organisations to share some of the profits of the sale. This list of specially selected providers has been drawn from the the places that the creators have used to make the items, so if they modeled it in Blender then they can decide to pass on a small percentage to them. It's all done via sliders, and looks like this.
The money will be drawn from Valve's take, which is lovely of them. And this has nothing to do with a Half-Life 3 ARG, btw. So stop looking at the pixels.