jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
RetroArch - RetroArch is a frontend for emulators, game engines and media players.
Z26 - Z26 is one of the best emulators for the Atari 2600.
Playnite - Source code generated using layoutit.com
Javatari - Javatari is a multiplayer Atari 2600 emulator written in pure Java with no external libs.
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.