Is there a List of Diablo 2 Mods that add quests.
Median XL is a popular mod that adds "uberquests" that involve tough new bosses, in addition to a whole bunch of other changes. It is also one of the few mods that still work with the latest version of the game.
I played Eastern Sun recently and really enjoyed the changes he made. It was both harder than D2 and yet easier in some ways. I won't spoil it but it has a Japanese motif which makes it different and he worked hard to change the areas and make things a little more convenient for players.
Zy-El is an older mod (1.09d is the max patch it works on) but it also has some appeal. I've been playing that recently and while it is uber tough (still in act one at level 32) it has some very interesting ideas and is a bit more loose in terms of it's fantasy setting. (Celebrity Posters? hmmm...) Anyway seems fun so far if a bit frustrating. Not sure how much of it is actually new quest wise but the first areas are bigger than vanilla.
(Both of these are findable at the Phrozen Keep archives: http://phrozenkeep.hugelaser.com/)