Mesen - Mesen is a NES/Famicom emulator and NSF player.
Z26 - Z26 is one of the best emulators for the Atari 2600.
Nestopia - A portable and cycle-accurate NES emulator written in C++
Steem Engine - Steem is an open source Atari STE emulator for Windows and Linux.
FCEUX - FCEUX is a Nintendo Entertainment System (NES), Famicom, and Famicom Disk System (FDS) emulator.
Hataroid - Atari ST emulator for Android based on Hatari.