The Unofficial Patch is not really a mod, but while the default basic patch only fixes bugs, the optional plus enhancements restore a lot of hidden content, including new quests and new maps!
I couldn't even play the game before I downloaded this for some reason, but now it works completely! I also love all of the features. It adds much more depth to the game, such as deciding what your character's history was. 10/10 because it deserves it! Keep it up, man!