Steam now offers early access to SpeedRunners, a four-player competitive platformer featuring grappling hooks and interactive environments. The game is coming to OS X as well, but it looks like early access is just for the Windows edition, allowing a discounted chance to play the game and to influence its development. As noted here the first update is already available, adding a new map. The tiny Build Games website has details and a trailer, and here's a primer on the game:

The objective is simple: Outpace your rivals by reaching the edge of the screen. You'll have to contend with wall jumping, sliding, obstacles, traps, grappling hook swings, and more as you dash towards victory, but that isn't all. With plenty of power-ups and weapons ranging from barrels to missiles there's more than just the terrain to worry about. A well timed attack is all it takes to propel yourself from last to first in this mayhem inducing racing game.