Of course, to be a decent tank, you should use all available tools that help make your job easier. Listed here are some of the most useful and important mods you can have.
TidyPlates, Threat Plates – This will replace the default blizzard nameplates to be much more useful and visible (and if you’re not using nameplates, you should be). The default settings work well enough, but you can customize the coloring and size based on how much threat you have on the targets around you. Very helpful in letting you see if someone’s about to pull aggro from one mob in a pack of half a dozen. It does get a little jumbled if you’ve got more than ten targets crowding around you but I can’t even begin to stress how important and useful this mod is.
Omen Threat Meter – Can’t be a tank if you don’t keep track of your threat along with those in your group.
LuckyCharms2 – A nice little toolbar that lets you mark targets quickly, as well as assign specific CC to certain marks. Beats marking everything by hand.
DeadlyBossMods - Very useful in raids, not quite so much in 5mans, but definitely helps out in announcing important boss events in a way that’s hard to miss.
OptiTaunt - This one isn’t a required mod, more of an informational tool for you and your group to explain why you didn’t taunt the mob that happens to be taunt-immune.
HudMap - I don’t use this much in 5mans anymore, but it can definitely help you learn situational awareness, letting you keep track of where everyone in your group is. Might be a little distracting until you get used to it.