99designs. Over 200,000 completed design contests. Over $85m paid out to designers. Spawned 30+ copycat businesses. Raised $35m from Accel Venture Partners, Michael Dearing, Stewart Butterfield and Dave Goldberg. Ranked in Inc 30 under 30 and BusinessInsiders 30 under 30.
I've also been honoured on the Forbes 30 under 30 list and Inc Magazine's 30 under 30.