LaunchBox - LaunchBox is a portable, box-art-based games database and launcher for DOSBox, emulators, arcade cabinets, and PC Games. Download it free!
itch.io - An online game marketplace and community.
RetroArch - RetroArch is a frontend for emulators, game engines and media players.
GOG.com - DRM-free game store, selling both new and old titles. No clients required.
Batocera.linux - Batocera.linux is an open-source and completely free retro-gaming distribution that can be copied to a USB stick or an SD card with the aim of turning any computer/nano computer into a gaming console during a game or permanently.
GnuPlot - Gnuplot is a portable command-line driven interactive data and function plotting utility.