Nestopia is an emulator for the Nintendo Entertainment System/Famicom (and Famicom Disk System). It is a computer program that acts like a physical piece of hardware. In other words, this is a method of preserving videogame history, with the welcome side effect of allowing you to play NES games on a modern computer.
Nestopia was started many years ago by Martin Freij, and quickly rose to fame for being one of the most accurate NES emulators around. Due to it being released as open source software, other people were allowed to enhance it. A Linux port was eventually done by R. Belmont, but the emulator, and the Linux port seem to have been abandoned since 2008.
Nestopia UE (Undead Edition) is a fork of the original source code, with
enhancements from members of the emulation community. This includes
support for new platforms, and bug fixes in the emulator core.