HGEC female body replacer - replaces the female bodies to make them look, well female. Its a nude body replacer, but has an underwear version in the downloads section. I'd post a link, but there is nudity involved, and I don't feel like linking to that here. Just do a search on TESNexus for HG EyeCandy. There are also armor replacers in the downloads section to have the game's armor match the bodies.
Akatosh Mount [http://www.tesnexus.com/downloads/file.php?id=10819] - Basically, it adds a ridable dragon to the game. A ridable, fucking dragon. I don't think I need to explain how awesome that is.
Lost Paladin of the Nine Divines [http://www.tesnexus.com/downloads/file.php?id=10505] - Not particularly special, but I helped beta test it, so yeah
Unique Landscapes Compilation [http://www.tesnexus.com/downloads/file.php?id=19370] - Changes the default Oblivion forests and scenery into truly diverse areas. Its really amazing and adds so much life into the game. The mod is an OMOD file, which means all you have to do is double click it, let it unpack, activate it in the Oblivion Mod Manager, and play! The alternative is installing each one separately, which is nice if you want to pick and choose, but if you want them all, go with this.
Elven Map Redux [http://www.tesnexus.com/downloads/file.php?id=3002] - Completely changes the map and makes it look much, much better.
Reneers Guard Overhaul [http://www.tesnexus.com/downloads/file.php?id=5977] - Makes the guards a lot better. Stops them from "sensing" crimes. And it makes the game better.
Better Amulets [http://www.tesnexus.com/downloads/file.php?id=4126] and Ring [http://www.tesnexus.com/downloads/file.php?id=4107] Replacers - It makes the rings and amulets look less like shoe leather and more like pieces of jewelry.
The extra mp3 music packs: Part 1 [http://www.tesnexus.com/downloads/file.php?id=4186], Part 2 [http://www.tesnexus.com/downloads/file.php?id=8033], Part 4 [http://www.tesnexus.com/downloads/file.php?id=4192], Part 5 [http://www.tesnexus.com/downloads/file.php?id=4189] - Yes, part 3 is missing, I can't find it. It may have been a type on the unloader's part, but I digress. This pack adds so much amazing new music to the game, and blends perfectly with the game. I use it and its amazing. Really helps immersion.