The Elder Scrolls series has a very strong modding community. The recently-released Morrowind Sounds & Graphics Overhaul v3.0 (FAQ) (trailer) (screenshots) is a mod compilation of 100+ mods (list), which gives a new lease of life to 2002's Game of the Year. It includes a semi-automatic installer (video) (forums) and options for the various mods. A gameplay pack has been in the works for quite a while, but if you want more than cosmetics and don't mind looking around, voila: you may dive in. However, the modding community is at its best in Oblivion. [more inside]
The Elder Scrolls II: Daggerfall Released as Freeware. The download (Fileshack) requires DOSBox to run; the linked article includes instructions for running the game under DosBox. Also related are DaggerXL, a game engine being developed to update / enhance Daggerfall for modern systems, and The Daggerfall Workshop.