Steam is currently the most popular digital rights management platform for Windows games, and have recently also started supporting Mac games.
Official logo (inverted)
Steam features in-game friendslists and chat options, achievements and other community features. In the past, Indie game bundles have appeared in the store for a limited time, bundling 5 or 10 games for very low prices (like 3-5 dollars per game on average).
It has in the past been notoriously hard to get an indie game onto the Steam platform, but apparently most Category:IGF winners and nominees have had it easier. Windows games are still more popular in ratio, but there are now quite a few Category:Flash games on the platform, as well as one MMF2 game, namely Saira.
As of this writing, no Category:Game Maker games have been distributed over the channel. A reason could be difficulty in incorporating the Steam code into the games.