Mesen - Mesen is a NES/Famicom emulator and NSF player.
FCEUX - FCEUX is a Nintendo Entertainment System (NES), Famicom, and Famicom Disk System (FDS) emulator.
Nestopia - A portable and cycle-accurate NES emulator written in C++
puNES - PuNES is a high accuracy NES emulator.
My Nes - My Nes is a portable open source NES/FAMICOM emulator written in C#.
NESBox - Nesbox Emulator is a multiplatform emulator of video consoles.