http://www.dota2.com/international/compendium/
The compendium has stretch goals, like projects on kickstarter. When more people buy the compendium ( its 10$, 25% goes to the prize pool) , the compendium reaches a certain stretch goal, and you get an extra ( like a courier or an HUD or whatever ). So if more people buy it, the compendium owners get more cool free stuffs.