Speedrunners is out of early access and is now officially available on Steam, offering a multiplayer running game for Windows, OS X, and Linux. Here's the premise of what's been described as a competitive Mario game:

In a city filled with superheroes, getting to crimes becomes a competition on its own. Fortunately there are enough rockets, bombs, grappling hooks, spikes, and other goodies lying around -- to make the competition fun and fast. Welcome to SpeedRunners.

SpeedRunners is a 4 player competitive platformer with grappling hooks, power-ups, and interactive environments. Run, jump, swing around, fire rockets, grapple onto people to knock them off screen.