These are the final addons I use in my Tankadin UI.
Curse you, Perry the Platypus (Decursive)
If you don't get the reference, it's from Phineas and Ferb. It's a really fun show that I watch with my daughter. Like all great kid's shows, it throws in some humor for the adults as well.
Decursive is a little addon I have to help me, well, decurse. Of course Paladins can't decurse, so it helps me de-magic, de-disease and de-poision, but decurse just sounds better.
Basically what happens is that when one of my team mates gets an afflication that I can do something about, their box will light up and a counter will start counting up the number of seconds they have whatever poison, disease or magic debuff. I left click on their box and that will cast Cleanse on them. This is very similar to using a Mouse Over macro, only all the mouse over stuff is built in.
I first started using Decursive for tanking Steelbreaker on Iron Council to deal with Fusion Punch. Now it sees most of its use on Faction Champions.
Heal me, Quickly! (Healbot)
I only break out Healbot on Firefighter and Lord Jarraxxus. These are two fights where I'm Prot spec/Prot geared, but I still find it useful to throw a heal or two. Healbot gives me a nice interface to do that with, and also gives me out of range indicators.
Talk, talk, talk to me (Prat)
Prat is a mod I use to modify my chat and enable me to scroll through my chat box by using my mouse wheel. Prat also shows me the level and class of each person as they text. It's one of those addons I found very hard to live without.
Money, money, mon-ey (Auctioneer)
There are parts of the game Tanks need to be good at outside of raids. One of those is the Auction House. You know the repair bills are coming and you know they are going to be large.
I have a dedicated bank alt that I send all my stuff to that I don't need. As I'm flying around, I'll spot a Titanium vein or pick up some Frostweave cloth off the mobs I killed. I send it to my banker who lists it with Auctioneer. He'll send the proceeds back to Honors. It works pretty good. I find the auction house positively boring and the thought of spending even a half hour posting stuff is not something I enjoy. Auctioneer makes it pretty quick and painless.
Organize Me (Bagnon)
A good tank carries around multiple sets of gear. I have stam sets, block sets, resist sets, avoidance sets. Then on top of that I have PVP sets, and Ret sets. I'm even working on a Holy set (but don't tell anyone I told you that, k?). That means I need a lot of bag space and some way to keep it all organized. Now I use the ingame Equipment Manager to manage my sets, but I use Bagnon to keep my bags straight. Bagnon simply gives me all my bag slots in one interface.
I can also use it to see what I have in my alts bags, which allows me to store my gems and enchant mats on my Banker.
Step in the ring (SSArena Frames)
One of the issues with Spartan is that I can't see the Blizzard UI Arena frames, so I needed a replacement. SSArenaFrames was what I found. It simply replaces the Blizzard Arena Frames and allows me to target the enemies in the Arena. However, I'm not doing Arenas as much this season so I haven't had as much need for it.
Wow, that was much more involved than I originally thought it was going to be. No wonder most people just give you a list of addons and pretty much expect you to figure it out.