-MonkeyQuest: Before I started using this one, I wondered exactly why it even existed. I thought WoW had plenty of quest tracking ability built into the default UI itself. But after using MQ for a while, I've realized it's much more than that. Sure, it's got a nifty little window that lets your track and sort your quests, but MQ even invades your inventory-- it'll tell you on an item's tooltip that the item is a quest item and exactly what quest it's for. Ditto on mobs' tooltips-- can't tell you how much time that's saved me, to be walking along and realize, thanks to the tooltip, that I almost walked past an invaluable quest kill. Don't get me wrong, I love grinding, but if you're questing, bring the Monkey along.
-DamageMeters (I use this one integrated with TitanPanel) Of all of these addons, this is probably the coolest one I've ever seen. At the end of an instance, have you ever wondered who in your party did the most damage, or the most healing, or took the most damage, or casted the most spells, or about a dozen other stats? DamageMeters tracks it all, and you can even output (numbers and percentage) that info directly into party, guild, or raid chat (in fact, you've probably seen the output at some point already-- I know almost every time I finish a successful run, someone asks me to see it). I also use it to determine what weapon works best or fastest, and even (as Shaman) what buffs help damage and aggro more or less. Unfortunately, it looks like the author of this terrific addon has quit updating it, but it still works with version 1.11. No telling what will happen with 1.12, but I've heard good things about SW Stats...
-Auctioneer: I mentioned this one when I posted about making money, but it's worth mentioning again, for the reason I listed then. If you want to make cash (or just learn more about how the economy works in WoW), install this addon. Install it, and scan the auction house, right now.
-CT-RaidAssist: If you're raiding at all, this is the UI you should have. In fact, if you're raiding with any regularity, you probably already have it. CT-RaidAssist (part of the CT mods, named after creators Cide and TS) lets you watch and recast buffs, monitor Main Tank activity, control and run raids, and pretty much everything having to do with having more than five people in your immediate party. You won't use it much if you haven't hit 60 yet (unless you have a nice guild that lets you tag along), but once you start raiding, it's invaluable.
I know there are lots of other addons out there, and I've even tried a lot of them (including Scrolling Combat Text, which seemed a little too distracting for me-- can't wait to see what Blizz does with their version of it, and Gatherer, which I liked but never really used-- I'm not much for professions in general), but these are the five that I can't play without. I know, however, that there's some you think I'm crazy for missing. Of course that's why we have a comments section, people! Which other addons would you recommend?