Temple of Elemental Evil: Circle of Eight
Infinity Engine: Widescreen Mod
Dark Souls: DSfix
Jagged Alliance: 2 1.13
Gothic 2: Vurt's Graphics Overhaul
Thief 1 & 2: NewDark
Guess those are the ones that come to mind, but I mod most of the games I play so I'm sure I'm forgetting some major ones, and I didn't bother listing some of the smaller (yet often essential) mods I enjoy. I have modded games myself in minor ways as well, but it's getting harder to do that these days, since more games ship entirely locked down.