With Daggerfall, it's difficult to say what exactly consists a mod and what not. I have picked six that have become somewhat classic and are not mere quest add-ons. If a homepage exists, I have linked it, otherwise you can download it here.
As yet, I have not played any of these mods. I did check out the altered or added graphics with Gavin Clayton's Daggerfall Explorer.
The first Daggerfall mod ever, created by Andrew Polis. It added stronger enemies, including knights on horseback, for which graphics had already been created and included on the CD, though they were not used in the game, and also some new graphics of Andy's own, especially a cool (and very un-medieval) black leather armor for female characters and lots of new helmets.
A mod by Tom Oldani that lets you play as a Daedra. Besides using many of Andrew Polis' graphics from Andyfall it adds some graphics of its own. It is a comparatively small mod in download size.
Though this mod did not add any new graphics, its download size is three times that of Andyfall or Kirafall. The reason is mainly the huge soundfile.
A "cosmetic" mod by Marty Runyon, less a Daggerfall but rather an Andyfall mod, mainly adding a white dress for female elves and changing the skin color of dark elves.
This mod, created by Donald Tipton and the smallest in terms of download size, attempts to make playing as a lycanthrope more interesting.
Two replacements of the text.rsc file that change most of the dialogs and messages, adding a lot more variety. Brendan1.rsc features lots of self-referential humor, while Brendan2.rsc is the serious "straight" one.
Faces for wood elves, high elves, and nordic women. These faces are not compatible with helmets.
Apart the mods, there are many minor patches that add a class or tweak some enemies. With Andypic, it is fairly easy to add custom graphics. You will find some at A.C.Crackpot's Daggerfall Hex-Page and at the Daggerfall Apparel Catalog.