One quest that hasn't been listed anywhere in this discussion (and I don't know HOW you people forgot it!) is Kragenir's Death Quest. It adds over 100 small quests that are totally integrated into the game (all of them use vanilla NPCs), along with a big main quest. It's not completely lore-friendly, but if you're looking for a little humor, you should check it out.
Also: Verona House Bloodlines (large post-MQ quest that includes a big house); JQ Assassin Quest (for assassins, duh); and NNWARE (11 new and altered Ayleid ruins, with a couple quests for high-level PCs).
I'm also trying out Return of Shadows, but I don't know if it's any good yet - I haven't gotten very far.