Without further adieu, here’s my list:

Eavesdrop – Eavesdrop is a combat log “replacement”, if you will.  It is absolutely fantastic for seeing incoming damage, incoming heals and outgoing damage in a very simple and easy-to-read format. Gives you a nice little summary after a fight has completed and it stores information for a long time so you can go back and see why you died. Was it a series of crushing blows?  Didn’t get heals?  Someone step in Flame Wreath? Remember that you can shift-click on a line and paste it into raid chat.  I cannot recommend this addon enough.

Omen – You all saw this one coming but I have to write it up anyway.  Omen is a threat meter so you can see just how quickly the DPS is catching up and it’s perfect for fights like Void Reaver or Bloodboil where you need to know exactly where everyone’s threat is at the time.  Do not log into WoW without this addon.

MetaHUD – I have used MetaHUD since I started playing and it’s invaluable for seeing your current health level, rage level and the health and mana level of the mob you’re tanking.  On top of that it shows cast times for special abilities of what you have targeted. Highly configurable, you can make it as big or small as you like and space it out anywhere on your screen.

Recount – This is one of Starfish Lovesong’s required raid addons but the thing that I like the most about this is you can make a separate screen with total raid DPS. It’s heartening to look over and see just how hard the rest of the raid is working to kill whatever you have stuck on you.  If you’re a raid leader that information is also valuable in determining how you are performing versus the last attempts.

Outfitter – I don’t actually care if you use Outfitter or not, but get yourself some kind of gear changing add-on pronto.  As a tank you’re going to be carrying around a lot of gear; your Effective Health set, your avoidance set, your threat set and your DPS set.  Instead of changing out pieces individually just setup a gear set and swap immediately.  Leave single-swap outs for your trinkets.

See also: ItemRack and Closet Gnome.  I have tried all three of these mods and decided that Outfitter was the best for me, your mileage may vary.

XRaidStatus (XRS) – This add-on serves two purposes: the first is that it enables you to chain-pull like a champ; a quick glance at the healer mana and you know whether you should pull or not.  The second reason you should pick this up is if you are a raid leader you can see if you should call a wipe or not based off of total dead, healer mana, total raid health, etc.  Both of these add up to being able to save you some significant time during raids.

WoWMatrix – This isn’t an addon at all but it’s an addon manager.  This little bastard will go out to every imaginable location to see if there are updates for your addons and then update them all at the click of two buttons.  I have over 300 addons installed so this program is absolutely critical for me.   As of this writing WoWMatrix supports 1,500 different addons.  Go get it.  Now.