Cobalt is a sidescrolling shooter that's getting a finished release on February 2nd, and between now and then its developers are running Bet On Cobalt. It's two weeks of non-stop Cobalt matches between teams of bots, around which the Twitch audience can gamble with fake money on who they think will win.
The bots are playing a mode called Team Strike. Viewers are given 100 'volts' on which to place to bets as to which team they think will win, by typing "ibet blue 50" or "!bet blue 50" and so on into the Twitch text chat. You can also go further by betting on particular achievements happening within a round, such as either team scoring a combo or a single bot taking out the entire opposing team. Bet well and you'll win more volts with which to bet more.
Or with which to buy things. Although volts have no real money value, they can be used to modify your profile within Bet On Cobalt, to name a bot, make a bot speak, or to trigger "special events". Then, if you're really successful, you're in with a chance of winning "real-life" prizes on February 2nd at 12pm CET. There's more details about all of this, including more on how to bet and how to win, over on the Mojang site.
Twitch phenomena seem to rise and fall in the time it takes Bob Ross to paint a friendly tree, but the one I still watch is Salty Bet. That stream pits bot-controlled characters against one another in heavily moddable fighting game engine M.U.G.E.N. It's funny, owing to the odd match-ups of anime girls, Street Fighter characters and, to pick an example on my screen right now, Dr. Robotnik from Sonic, as well as being oddly compelling because it's rarely obvious who will win. It's the most likely inspiration for Bet On Cobalt, although I'm not sure that Cobalt - with its teams of more or less identical little robots - has the same lasting appeal to me as a spectator stream.
I do like Cobalt a lot, though. It's been a long time since I played it, but there's a lot of finesse to it; I especially like how near-misses trigger slow motion and how rolling allows your character to deflect bullets. It made for some shouty, dramatic matches when I played it in local multiplayer a few years back.