Way back in the early days of First Person Shooters (FPS), aka 3D gaming, we had a little title called Quake to keep us interested and playing on computers. Since Quake was quite open, much like how id Software did with Doom, people were able to make mods, new characters and changes to the engine itself. Quake was heavily modded over the years by fans. We saw kart racing, airplane games, etc see release by fans. Commercial companies wanted in on this openness too though. X-Men: The Ravages of Apocalypse was the first expansion pack that featured licensed characters as popular as the X-Men from Marvel.
Expansion pack is probably a misnomer though as X-Men was a total conversion, similar to the kart and airplane games mentioned earlier. For those that do not know, a total conversion is basically taking one game and changing things around so that it is a new game. God of War on the NES took Rygar and made a new game for instance, that is a total conversion. X-Men: The Ravages of Apocalypse is billed as a total conversion but it falls short a bit- it is still a 3D FPS and the game is easily recognizable as using the Quake engine.
This freeware release of X-Men: The Ravages of Apocalypse features a few additions that the retail release did not. There are bug fixes that Lord Havok have implemented, this release is compatible with Lord Havok’s DarkPlaces engine and you can even play this on your PSP using PSP Quake. No word on if it is compatible with the Sega Dreamcast, which has had some really great Quake ports.
X-Men: The Ravages of Apocalypse is released as freeware by Zero Gravity Entertainment, the original developers of the total conversion. This freeware version of X-Men: The Ravages of Apocalypse does not require a retail copy of Quake to run.