Either I completely missed that aspect of the good old days, or they just never existed.
The amount of mods, addons and gamemodes added by Epic for Unreal Tourmanent 1 is a good example.
All right, that would've been the time period I was essentially completely away from gaming, so it's possible I actually did miss them.
That, or everybody in this thread will be able to put ~5 examples together, after thinking really hard.