Stream Racer is a racing game for Twitch.tv streamers. Host your own races, stream it on Twitch.tv and have your viewers join you. Engage more with your viewers by giving them a racing car on your stream.
Basically anyone can get the game free on steam and host their own races if you're a streamer yourself or you can just be a viewer and participate by racing!
If you're watching someone who is playing then just type race to join during the lobby and you're all set. Once per race you can use the command !boost to speed up.