Wine - Open Source Software for running Windows applications on other operating systems.
Nestopia - A portable and cycle-accurate NES emulator written in C++
CodeWeavers CrossOver - Don't buy a Windows license, don't reboot and don't use a Virtual Machine. Try a free trial of CrossOver to run your Windows software on Mac, Linux and Chrome OS.
FCEUX - FCEUX is a Nintendo Entertainment System (NES), Famicom, and Famicom Disk System (FDS) emulator.
PlayOnLinux - PlayOnLinux will allow you to play your favorite games on Linux easily
puNES - PuNES is a high accuracy NES emulator.