The Knights of the Old Republic 2 mod which restored and polished a lot of content originally cut from the game. An amazing amount of work went into it.
http://www.rockpapershotgun.com/201...gonna-kreia-up-kotor-2-completed/#more-117715
I believe thanks to this mod Kotor 2 is finally available digitally if not at least it was a huge conincedence. Definitely one of my favorite mods.
This Torchlight II mod adds new classes, enemies and so much crap to the game.
Civilization Nights Gods and Kings is another great mod that has pretty much become a must have for me when playing Civ V.