FCEUX is a Nintendo Entertainment System (NES), Famicom, and Famicom Disk System (FDS) emulator. It supports both PAL (European) and NTSC (USA/JPN) modes.
NES emulator and NSF music player for Windows..
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 is an open source Win32 NES emulator written in C++. The original goal was to emulate the NES down to its hardware quirks.
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.
The source is available on GitHub.