LaunchBox - LaunchBox is a portable, box-art-based games database and launcher for DOSBox, emulators, arcade cabinets, and PC Games. Download it free!
Ubisoft Club - Ubisoft Club app enables you to get updates about your favorite games timely.
GameEx - GameEx is considered to be the most powerful and feature rich front-end for MAME, GameBase, Daphne, PC Games and all command line based game emulators, along with being a complete Home Theatre PC solution.
itch.io - An online game marketplace and community.
GamersGate - Buy and download games instantly for PC and Mac.
GOG.com - DRM-free game store, selling both new and old titles. No clients required.