Knights of the Nine
Price $9.99 USD
The Knights of the Nine is the most expensive plug-in for Oblivion, but this is because it delivers the most content. With a whole new quest series, faction to join, set of armor, and brand new locales it's a substantial edition to satisfy any Oblivion fan's thirst for more content.
Spell Tomes
Price $0.99 USD
A simple add-on Spell Tomes adds books into the world of Oblivion that teach spells simply by reading them. nice for spellcasters, but not worth it for most others.
The Vile Lair
Price $1.89 USD
The Vile Lair expansion creates the new location Deepscorn Hollow. This place is designed for an evil character and is the first real home base for vampires. It features a healing stone that can cure vampirism even for the infamous, a depiction of Sithis to worship, a human for those times when you're thirty, and a minion who you can send on a killing spree. All this makes a nice little place for villainous characters to stay.
Mehrunes' Razor
Price $2.99 USD
Mehrunes' Razor adds a new dungeon to Oblivion larger than all the other ones on the original CD it's simply massive. There are 17 new items and 12 books hidden within and a new weapon that has the potential to kill in one hit.
The Thieves Den
Price $1.89 USD
A new pirate ship is added to the game that makes an excellent hideout for thieves and assassins. Full of minions, fences, and trainers it's the perfect place to equip your master thief.
Wizard's Tower
Price $1.89 USD
Surpassing even the Arcane University the Wizard's Tower is the best place to train a spellcaster. There is a garden and laboratory for alchemists as well as an enchanting and spell making station. There are warps to every Mage's Guild Hall in Cyrodill and the view from the tower is the highest point in all of Oblivion.
Orrery
Price $1.89 USD
An entirely new quest is opened up to retrieve mechanical pieces from bandits. Once they are collected you can reconstruct the Orrery at the Arcane University. This massive machine projects stars and planets onto the ceiling like a super planetarium. Beyond looking at this marvelous effect you can use it to influence stats by looking at the stars.
Horse Armor
Price $1.99 USD
A purely cosmetic add-on the Horse armor is exactly what it sounds like, armor for your horse. If your horse is often fighting then this will help, but for the most part it's for looks.