TinyBuild's SpeedRunners has been out for almost 4 years now. The 4 player racer/platformer will be free on Steam over the coming weekend. Odds are good you might get hooked if you decide to give it a try.
Long time party favourite and indie hit, SpeedRunners will be on Steam over the coming weekend. It's fun to play online, but really shines in the way it can break life long friendships in 4 player couch races.
Grappling hooks, power ups, speed boosts, and everything you love from Mario Kart or Crash Team Racing - SpeedRunners has it in platformer form.
The game is still populated quite well considering how long has passed since launch, with the Steam version boasting an average of about 400 players at any given time, so there are enough players to race if you have no friends on your couch at the moment.