Most Helpful Guy
Pretty much anything that became Open Sourced or had their Source Codes released and had shit tons of free custom made goodies from their respective fan-bases.
Such as free new levels, music, sounds, graphics, gameplay tweakes and all sorts of "Mods" or modifications.
Examples are the Classic Doom games, Duke Nukem 3D, as they all have free custom level creators and various mods support and Source Ports since their Source Codes had already been released.
I missed those days since now almost everything is focused entirely on the DLC "Microtransactions" model and platform.
Not many custom levels and or map editors/creators get released any more for most games and even less will have their Source Code released 😢 😢 although some games will occasionally get a SDK release.