I also just found this:

https://github.com/loicreynier/awesome-modding

basically just a huge compendium of everywhere all kinds of mods for anything are hosted, that'll give you an idea of how the game modding scene is actually rather dispersed, not only monopolized by nexusmods.

not sure if its in this huge list but:

fpsbanana

is another one i am quite familiar with, been going strong with mostly source mods... possibly since the late 90s, at the least the early 2000's.