Nestopia runs as an open source emulator. It emulates the Nintendo Entertainment System (as well as Famicom hardware) in a very accurate manner. The emulator is written in C++ and was initially released in 2003, with a final version hitting the Internet in 2008. Obviously, the project has been abandoned, but the program still does a fairly good job and can be found over the Internet for free. It operated on a few different operating systems – Linux, Windows and Mac OS X.
When it first came out, Nestopia had some high requirements – way higher than most of its competition. For example, it required a modern 800MHz processor. But then, these requirements were mandatory for a smooth and accurate emulation. Moreover, the emulator allows customizing graphics, colors and sounds. While it may have fewer features than a few rivals, it is quite complete and lacks major bugs, so it makes a great choice for NES enthusiasts.