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.
A lot of bugs have been fixed and improvements have been made already. The emulator core is in pretty good shape already, so most of the improvements will be related to user-friendliness and modernity. Support for FreeBSD, OpenBSD, and NetBSD is now integrated. The Windows version is also being maintained, and will receive any updates to the core that may come, such as support for more mappers. There is also a libretro port that can be used on most (all?) of the libretro-supported platforms.