I stumbled across an interesting tool someone wrote to encourage interaction between the people watching a stream and the person streaming a roguelike game. The website is a wrapper around a standard Twitch stream that lets the people watching bet fake currency on where and how the streamer is going to die. (Roguelike games are ALL about dying with permanent death!) When the streamer dies they go to the website and enter in the details of the death and the website pays out to the people who guessed correctly a split of all the currency bet across all options.
I found it very interesting. I didn't know much about the game being played (Crypt of the NecroDancer, a roguelike where you need to enter your commands to the rhythm of the background music) but it was extra fun to watch the stream because I was betting on the outcomes. I got it right once when I bet that he'd die to a red dragon. Woo! (I bet on the red dragon every time... It only came home once, but at least it did come home once.)
I'm constantly amazed at extensions to let viewers participate in a stream. This is no Twitch Plays Pokemon, but it's still pretty cool.