Needing Cobal or Obse and sometimes both:

Deadly Reflex, Toasters Share and recruit. The original one is still available and I like the quest idea but the latest one has so many related things you can do. Quals Harvest is fun. FF_Real_Thirst I love those potable wells. Redecoration, a dangerous mod but one I can't do without. Just be sure to read the read-me.The latest Keychain requires Obse.

I'm a big fan of quest mods, especially those that pay close attention to lore.

Of course all of the DC/CUT mods.

Integration (http://www.tesnexus.com/downloads/file.php?id=22320) is fun. Many small quests, all of them related to bits of Elder Scrolls lore. Requires LAME (one of the best magic overhauls) and the Race Balancing Project, but both mods are very good in and of themselves.

Heart of the Dead is an incredibly good quest mod, one of the best I've seen.

Lost Spires. A classic quest mod, lore friendly and very polished.

I'm just starting to get into some of the new DC mods (Fort Akatosh, Guards of Cyrodiil), and I'm really liking what I've seen so far.

Daggerfall Memories - Cybiades (http://www.tesnexus.com/downloads/file.php?id=22320) - This one will give you a grin if you remember Daggerfall, and even if you don't it still introduces some well done dungeon crawls and a few very memorable locations.

Other great stuff has been mentioned in this thread (Glenevar Castle. Duke Patricks mods). One of the great things about Oblivion is that there are always cool mods you haven't tried.

Anyone buying Benirus Manor MUST have this mod and I seldom use the phrase 'must have'.

Better Benirus Manor by Jhettienna at TESnexis

http://www.tesnexus.com/downloads/file.php?id=18054

I won't spoil it for you by describing it but it is fantastic. It is not a quest mod but much more than eye-candy and what good eye-candy it is. I am tempted to say it is perfect. It comes in COBL and non-COBL versions. Of course I have COBL, don't you?

I would agree that Heart of the Dead is a great mod, and while somewhat lore-friendly, it takes liberties when it comes to TES fact but nothing way out there. Again though, it's a rather large file.

TNR (Tamriel NPCS Revamped) is an amazing mod that changing every single face in-game to be well, simply awesome. Races are more unique, and each NPC has they're own unique face that suits there personality. Must have, and somewhat of a small download.

Tamriel Travellers is also pretty good, as i always hated how the roads had no one on them.

Other than that, I can't really think of any other small mods that havent been mentioned above.

Oh, you ever mentioned that you had COBL, and should definately have COBL.