Another great invention by the creators of Penny Arcade: Greenhouse.
Steam has done a great job making independent and smaller titles much apparent to the populous, and since titles don't have to compete for shelf space a genre for every palate can be made readily available. And while CodeWeavers has done their best to allow Steam & Source titles to run on Linux & OS X, it can't be said that Steam is a cross-platform solution.
Not so with Greenhouse. It offers native support for OS X, Windows and Linux in tandem. And their inaugural title will be cross-platform. And if they continue to support independent and episodic titles, this could be a bigger competitor to Steam than GameTap.