Studio Bean has just has just released the first game that uses Twitch's interactivity features as a primary feature. This project was funded in part by Twitch through KickStarter. At its core, Choice Chamber is a basic 2D side-scrolling game, throwing enemies and environmental hazards at players. But where things get really mixed up is in the fact that anybody watching the broadcast who can make live comments can have a direct impact on how the game unfolds.
UPDATE: This article previously suggested that Twitch developed this game when it was actually "Studio Bean." Twitch's role was to contributing money to their Kickstarter to help bring the game to final fruition.