http://www.tesnexus.com/downloads/top.php
The central resource for oblivion mods. Any mod in the top 100 that catches your fancy will probably be well enough made to be worth while. Ones of particular note are:
Deadly refelx: improves combat by making it more in depth, adds critical strikes, shield bashing, kicks, decapitations, slit wrists, slit throats, special sneak attacks, new combat mechanics. ect. It makes mealee a very involved experience.
Midas magic: hundreds of new, amazing spells. You can find youtube videos of it, but they're all very creative. It comes along with a very balanced, immersive, and fun way of acquiring new spells. a must have for mage characters.
FCOM: This just makes the game 100x better flat. hundreds of new weapons, hundreds of new enemies, (I'm being literal here) new factions, new leveling system, everythings been rebalanced...I just a name everything it does, it makes oblivion a whole new game. An overhaul mod in the truest sense. But it's a pain in the ASS to install, so be wary. Here's their site:
http://devnull.sweetdanger.net/convergence.html
Unique landscapes: Self explanitory really. And it removes most peopls main complaint about the game. Very well.