What are the "must have" addons for World of Warcraft?

a threat/aggro meter. Cryiron listed some useful ones, questhelper helps out allot

Gatherer (w/heads up display), Big Wigs, and Rating Buster. If you're someone who relies on Epeen meters, Recount is more accurate and less resource-intensive and crash-happy than Omen. Also, Bejeweled is an extremely well integrated add-on even though it's just for fun. It's a good time killer during flight paths and such.

Then of course you have some class/profession specific mods, like Enchantrix, Pally Power, and Necrosis (for Warlocks.)

Of all of these, I think that Gatherer is the most useful, well thought out, and well-tested.

I've grown to depend on questhelper if for no other reason than having become used to it over time. The problem is, questhelper is known to cause performance hits and you can even see one of them when it initially loads upon login to a char that has a full or near full questlog. In addition to Questhelper I use Lightheaded, which adds a pain next to the questlog with wowhead comments specific to the quest selected.

Gatherer I agree with, although gathermate is just as good and in some ways superior. DeadlyBossMods I think 'everyone' will agree with.

Recount and omen are commonly used in conjunction with eachother. I agree with ratingbuster as well.

One addon that's almost getting to be essential, at least since the advent of the faction transfers and the 'min dps over skill' scenario dominating pugging now (ie if you don't do x dps and yer gear score isn't x you aren't going) .... is Gearscore. It can also assist you in finding other chars of your class to look at on armory to compare specs/gear.

Finally ... if you do battlegrounds heavily enough, there's a cool addon called dgks (search for on curse). I turn all the notifiers off and only use it for the sound notification (which only you hear) for kills. Might not sound like much but it makes bgs a little more fun.