Team Fortress 2 community.
"Once the votes are in, we'll curate the top-rated entries, and the best of the best designs will be made available as official merchandise at the Valve Store and various online retailers," a post on the Team Fortress 2 blog announced. "You won't just be ensuring your place in history: You'll also get paid a handsome royalty for every unit sold." Check out the official FAQ for more information on submitting an idea.
Valve has been in the business of selling hats and other apparel for years ... and now you can, too!