Some folks are unhappy with how Steam has been lately, so they might be looking for some alternatives. Origin is an option, but now GOG is getting into the game.
Today, you can sign up for the open beta of GOG Galaxy. This client actually sounds like it has some really great features. You can turn off auto updates if you like. If an update does something to your game, you can roll back to a previous version. Backup your games or download a DRM free version as well.
Of course you'll get achievements, multiplayer matching, friend's list, chatting, and a few other of the game client staples. For starting it, it sounds very promising. Another player in the game is always welcomed and I can't wait to see how Galaxy evolves and competes with Steam and Origin.