Perpetrators: Ubisoft, EA, Microsoft
Worst Example: UPlay
You know what I love more than anything in the whole world? Having to run multiple game clients just to manage my library simply because a particular publisher thought they'd be able to deliver me a better experience by tacking on more shit for me to download and manage before I play my game. As much as letting Steam control the digital marketplace is bad for competitive business, having 6 different applications controlling my games is even dumber.
The stupidest moments come when you buy a game through steam and it loads up Games for Windows Live or some other nonsense as an intermediary before you can even play. Who the hell thought this would be a good user experience? Offering achievements, bonuses, or whatever else you're peddling is worthless when it hinders the experience of the user and that's exactly what these clients are doing.
The Solution: Ditch the clients altogether and sell your games as standalone executables.