Descent 1 and 2 (numerous singleplayer levels, although only a few of them were reasonably good, and a few weapon mods)
C&C: Red Alert (a TC that I still play fairly often, many multiplayer maps, two singleplayer missions, a complete desert tileset)
Tiberian Sun (some multiplayer maps, minor changes to existing mods)
Freespace 1 and 2 (a campaign, several standalone missions, many other odd things here and there)
Battlezone (a few maps)
Lemmings (8 custom levels, which I made only last year)
Train Engineer (several layouts for this old Mac game)
Streets of Simcity (a few maps, basically just SC2000 city files)
I was planning to get back into modding with C&C3, but that game is apparently very difficult to do much with compared to the older C&C titles, although basic multiplayer maps are fairly easy to make. Crysis might be another candidate, as it seems to have very good editing tools.