This list needs desperately to be updated.   So... update coming soon. 

Alchemy and Magic:
Advanced Magecraft
Advanced Mark and Recall
Arcane Archery
At Home Alchemy
RuneSkulls


Armor and Weapons:
Addonay's Classical Weaponry
Adonnays Elven Weaponry
Adonnays Ranger Coats
Alternative Light Armors
Armamentarium
Armamentarium - Armor
Armamentarium - Katanas
Armamentarium - Staffs
Elven Boot Improvement
Kalikut's Rare Trade Goods
Kafeis Armored Circlets
Seductive Male Dark Seducer Armor and Replacer
Lich Gear
MCs Staff Pack
RealSwords - Argonian
RealSwords - Orc
RealSwords - Redguard
RealSwords - Bosmer
RealSwords - Breton
RealSwords - Goblin
RealSwords - Khajiit
Shadow Armor - Pack 01
Shadow Armor - Pack 02
Shadow Armor - Sotonhorian
Slof's Oblivion Goth Shop
Slof's More Male Stuff
Slof's Oblivion Goth Shop II (currently down)
Slof's Sea Armour
Lighter Elven Cuirasses
Wood Bows and Leather Armors


Clothing:
JQ Amulets
Dwemer Spectacles and Dwemer Spectacles NPC Equip
Slof's Bracelet Collection
Slof's Bracelets and Multi-Rings
Slofs Male Clothing
Slof's Oblivion Robe Trader
Traveler's Cloaks


Creatures:
AliveWaters
Slof's Dogs
Slof's Oblivion Horses Base
Slof's Extra Horses Pack


Face and Body Mods:
Capucines Character Expansion
Djangos Unique Features
Robert's Male Body Replacer v4
Slof's Khajiit retexture
Slof's Oblivion Better Beasts (currently down)
Slof's Dremora retexture for Robert's Male
TFF FantasyFigures Armor Base
TFF FantasyFigures Clothing Base
TFF FantasyFigures Races Base
Capucines Character Expansion for TFF and Robert's Male Body
Cosmetic Compilation for TFF and Robert's Male Body
Slof's Better Beasts for TFF and Robert's Male Body


Houses:
Alchemist's Cave
Bravil House for Sale Upgrade
DandD Portable Hole
Misty Hideout
Summonable Private Quarters
Silent Stones Camp
Unfathomed Tower


Items:
Bag of Holding
Bedrolls
Cane of Sheogorath
Imperial Furniture
Nighteye Goggles


NPC:
Tamriel NPCs Revamped


Overhaul-type Mods:
Obscuro's Oblivion Overhaul
Martigen's Monster Mod
COBL
TIE4mods
StarX Vanilla Vampires Revised
CDEP: Underpall Cave


Quests:
Verona House: Bloodlines
Elvenguard
Thieves Arsenal: Odd Jobs Complete
Blood & Mud: Dirt Deluxe Anglais


Realism:
Real Hunger
Real Thirst
Real Sleep
Realistic Fatigue
Reneers Gold Mod
Unlimited Rings


Textures:
Book Jackets
Elven Map Redux: Castle Dunkerlore
Beaming Sunglare
Grimbots DLC Book Jackets SI
Kafeis Better Amulet Replacer
Kafeis Better Rings Replacer
Loading Screens Themed Replacer
Loading Screens - Add On and Dev Kit
LoadingScreens - Shivering Isles
Oblivion PolyGone Overhaul: Polys be Gone!
LowPoly Grass
Parallax Cities Giga Pack
Phinix Waterfix
Potable Pastiche
Slof's Sexy Male Armors
Slof's Sheogorath Outfit retexture
Vality Shivering Isles map


Town Mods:
Better Cities
Vaults of Cyrodiil


Tweaks:
Less NPC Conversations
No Merchant Barter Chatter
Initial Glow
No Quest Items
Denock Arrow
Dousing The Flames
Loot All Armor and Loot All Armor: SI
No More Annoying Comments About Your Skills
RD Better Book Collision
Robes over Armor Plus
SniperSneakSkillup
Better Hand placed Loot
Attack and Hide v2.1 bundle pack


Utilities:
OBSE
Streamline
Unofficial Oblivion Patch
Unofficial Official Mods Patch
Unofficial Shivering Isles Patch
Wrye Bash


Weather and Atmosphere:
All Natural
Real Lights
Weather Inside


Posing and Actions:
Actors in Charge
Actors in Madness
Actors in Emotions


Great places to find mods and other resources:
TESNexus
Planet Elder Scrolls
Official Oblivion mod forum
Slof's Hive