Are you intending on Raid Leading as well? (Tanking and Raid Leading just seem to fit so well together) If so, my list will expand, but just for tanking I would say....
Grid - At a minimum some sort of raid frames. I really like grid. You need to know whats going on with the rest of the raid.
TidyPlates - It is more useful at times than others, but I can't imagine tanking without it anymore. Threat plates is awesomesauce.
MSBT - Can show procs like Sword and Board, also gives you an idea of incoming damage and heals (by who)
Pitbull - Awesome unit frames addon. Can also add groups like Main Tanks or Main Assists. Can replace grid, but I prefer grids raid frames.
Satrinas Buff Frames - Specifically I have 2 additional frames for: my debuffs on my target (t-clap, demo, sunders etc) and my self buffs on me (Commanding Shout, wellfed, flask duration etc)