UltraHLE is a High-Level Emulator that made history on January 28th, 1999 when it was released as the first and only successful Nintendo 24 emulator.
Epsilon and RealityMan created this emulator. They, however, did not let anyone know what they were doing into they were completed with this project. They started with the hardest aspects and levels of functioning and from there tackled the easier aspects and functions working downward. Once the project was completed they distributed this technology to everyone across multiple sites.
This changed everything for the gaming industry. This was a new and first of its kind approach to emulation in the industry. It tried to capture what a game is trying to achieve by imitating that function. In other words, it was used as a simulator. This technology allows Nintendo 64 games to be played on a computer with or without a console.
It's famous for being the first Nintendo 64 emulators capable of running commercial games at full speed on the hardware.
UltraHLE has never had an update and was discontinued 6 hours after its initial release. The limitations of this particular HLE emulator would have prevented newer games from running on it. It works best with games that it’s been made for. Popular games played using this technology are Super Mario 64, Ocarina of Time and Legends of Zelda. This emulation paved the way for playable emulators of the current day.
Both RealityMan and Epsilon had a lawsuit filed against them by Nintendo for this emulation causing them to go silent. This also resulted in the emulator being shut down 6 hours after the initial release. Nintendo was ultimately trying to protect their intellectual property and stated emulators are illegal to make and to use.
The only way to replay any of these old games such as Super Mario 64 is by using emulation.