Javatari - Javatari is a multiplayer Atari 2600 emulator written in pure Java with no external libs.
jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
Mame - MAME stands for Multiple Arcade Machine Emulator.
Z26 - Z26 is one of the best emulators for the Atari 2600.
WinUAE - WinUAE is a Windows port of the emulator UAE.
Hataroid - Atari ST emulator for Android based on Hatari.