FCE Ultra X, typically shortened to FCEUX, is a Famicom (NES) emulator, as well as a Famicom Disk System (FDS) emulator. It is an amalgamation of several FCE Ultra forks, designed to bring the FCE Ultra branches to a halt due to the vast number and their differing features. Ironically, it has also been forked, though mostly to increase the amount of platforms it is on, such as a GameCube and Wii port known as FCEUGX.
Despite its extensive toolset, it is less accurate than other NES emulators like Nestopia, Mesen and PuNES. However, thanks to FCEUmm being one of the forks it incorporates, it still has its uses as a highly compatible standalone emulator. It is recommended to download the latest SVN build as the stable release is very old. It is also popular for developing new NES games, due to its excellent debugging tools, however if you develop games/ROM hacks, be sure to test them on more accurate emulators, and/or with a flash cart on the real system to ensure compatibility.