Emulators
Fceux
FCEUX is a Nintendo Entertainment System (NES), Famicom, and Famicom Disk System (FDS) emulator. It supports both PAL (European) and NTSC (USA/JPN) modes.
RockNES
NES emulator and NSF music player for Windows..
Nestopia
Nestopia is an open source NES/Famicom emulator designed to emulate the NES hardware as accurately as possible, and is widely considered the most advanced cycle-accurate NES emulator.
Nintendulator
Nintendulator is an open source Win32 NES emulator written in C++. The original goal was to emulate the NES down to its hardware quirks.
iNES
iNES is a program that emulates Nintendo Entertainment System (NES) and Famicom videogame consoles on your computer. It plays NES games on PCs, PocketPCs, Macs, Unix boxes, etc.
JSNES
JSNES is a port of vNES to JavaScript. The video is output on a canvas element, and the sound is played with dynamicaudio.js.
The source is available on GitHub.