Curse

ArcHUD3- very useful to keep an eye on your hp/mana, numbers and percentage closer to where you will be looking at your spells.

AtlasLoot Enhanced- ideal in looking for gear upgrades, know what bosses your gear drop from.

Bartender4-  a MUST have to organize your spells, resize them, hide on combat if you wish, put your xp/rep bars somewhere useful.

Chatter- another useful one to get rid of unsightly blizz tabs on chat frames, resize text, use different fonts, customize colors etc.

ClassTimer- very useful for watching your DoT timers, buff timers etc. I put these right next to my HUD, don't need to look up at my raid frames at all when dpsing.

Deadly Boss Mods- want to know when you're standing in something you're not supposed to, not standing in something you're supposed to, when to push buttons, and what bosses are about to do? Then don't look further, this is the addon for you.

Elkano's BuffBars- blizz buff/debuff frames are big and ugly, this addon allows you to make buffs/debuffs small, have timers, even names if you want.

Grid- I like to use this one still for my healers, makes unit frames small and all in one place, shows incoming heals, health lost, healing spells on individuals etc. Though the new blizz raid frames aren't half bad, they are missing alot of the customization of this addon.

kgPanels- gives a nice solid background for chat frames, just a solid box you can make to size and colored how you want (basically).

MikScrollingBattleText- another good one to see what dmg you're doing, damage you're taking, heals you're performing, and receiving, I also have this right near my HUD, another thing you don't need to look up for...

Omen Threat Meter- Useful in knowing how close you are to taking aggro off of tanks, or if you're a tank, let's you know you need to step it up if dps are stealing threat, you can see who's doing it and by how much.

QuickMark- very useful for ppl who lead raids, tanks or ppl who just like to see marks on targets, a small bar that has all raid markings right on it.

Random Mount- you want to be able to show off your mountain o' mounts right? This one randomly selects your mount each time you mount up.

ReadyCheckAnnouncer- want to know who's ready and who isn't?  This one automatically calls out (after you /readycheck of course) in chat if everyone is ready, who's afk, and who isn't ready.

Recount- shows top dmg doers, receivers, heals done, heals received, dispels, and many others, very nice if you want to know who's the best and what they're doing.

SexyMap- I mean this one...it's in the name...make your map sexy, hide buttons, literally customize your map to the fullest.

Titan Panel- Shows such things as gold, framerate, xp per hour, xp to level, clock, volume, durability of items, and many other things.

_NPCScan- just neat, tells you when you're near a rare and marks it for you.

I'm actually using the blizz unit frames right now for the most part, except of course grid for healing, they've gotten alot better and the unit frame addons out there aren't very good, at least I don't think they are, there used to be some good ones, but haven't been in awhile.