Thanks for the excellent replies, everyone!
If you want any further help I suggest you tell us about yourself; what you like and what you're looking for... Here's some questions to get you started:
Have you played Oblivion already?
What sort of things interest you?
What type of Character do you usually play? (good/evil, theif/mage, roleplayer/random-no-story-hack-and-slasher?)
Are there specific types of mods you think would interest you? (Quests, graphic mods, gameplay mods, overhauls, etc.)
When did you leave?
1. Yes, I played through Oblivion when it came out-- I played with an Argonian, and made it my goal to complete every quest/guild in the game. I think I was level 34 when it was all said and done. Nothing like bandits in full daedric armor, eh? They should be grateful.
After beating Oblivion, I went back to Morrowind.
2. Well, I'll list my favorite Morrowind mods, and see if you guys can give me any sort of guidance.
~Suran Underworld
~Balmora Expanded (particularly Balmora Underground)
~Carnithus' Armamentarium
~The Underground (1 and 2)
~Wizard's Island
~Horror Mod
~Glory Road
~Vampiric Embrace/Hunger
~Illuminated Order
I know that I went through both Suran Underworld and The Underground twice, with two separate toons.
3. I tend to really enjoy darker characters, but generally games equate darkness with violence-- and once you are overpowerful (as I felt I became in Oblivion), it's tough to find it gratifying. I enjoy more of the seedy/gritty/darker side of things in Elder Scroll games. I'm a huge roleplayer-- the Vampiric mods made gameplay tough (having to feed, stay out of sunlight, et cetera), but if I didn't play so many vampiric characters, I would use Necessities of Morrowind.
4. I'm interested in whatever, but anything that greatly contributes to roleplaying or new guilds/quests/roleplay without breaking things (Suran Underworld did a great job of this) is ideal.
5. I left Oblivion mid-2007, Morrowind (second time) around late 2008.
Thanks!