People competing in multiplayer video games. With professional commentators, multiple camera angles, replays, custom graphics, sponsors, players that earn a living playing on professional teams with coaches, and everything else you'd get on ESPN watching a football game. I don't think any sport-themed games are played professionally though.
At the time of my comment, Twitch is live broadcasting the Hearthstone World Championship, Duelyst Pro League, Vainglory Evil Eight, Starcraft II World Championship Series Global Finals, and SMITE Pro League.
eSports tournaments have paid out over $100 million in prizes. Some of these tournaments are being carried live on cable channels in other parts of the world where they're more popular.