First of there is Omen, omen is a Threat meter and Yes I have use KTM(KLH). But I have found that Omen updates much faster and has proven to be more accurate on a number of occasions. Next Is some type of Damage/Healing Meter. Currently I use Recount, it is out of date but has not broken yet. The reason I use Recount is not only does it Provide Dmg/Healing/Over Healing It also records who breaks CC, Where your dmg is coming from, How often you miss, Where you take dmg from and what ability that mob is using. It really breaks everything down so you can disect it and figure out where you are having problems or where you can improve (which reminds me to post another topic about DPS meters and how to NOT use them).
Everyone who has ever so much as looked at the Auction House should download Auctioneer, it's far to handy not to have. If your sick and Tired of Alt-tabbing to look at drops off bosses you can't go wrong with Atlas Loot Enhanced.
Another must have is a coord addon, the one i currently use is Location Fu, I know it doesn't list the cords as 00.0 , 00.0 but common if you cant find something without needing to go to the tenth degree of a cord you need to just quit and go play Hello Kitty Island Adventure or something. All of the FuBar addons are great in my opinion pick and choose your own at will you cant go wrong, they are updated very often and even if they are not they don't seem prone to breaking easily.
Finally a new little Gem I found recently is Quest Helper, this little buddy uses some free ware databases and will actually make way points on your map for your various quests and plot a suggested path to take to do as many as possible and then return and do mass turn ins so as to reduce the amount of total run time. a warning that it does miss some quests but by and large It does help reduce questing time in my opinion.
And last but not least, something no raider should ever have to live without Ratingbuster this little guy is a god send, it looks at a piece of gear you mouse over vs. what you are wearing and it tells you what you gain vs what you lose it calculates all stats down to HP/MP/+DMG/+Crit blah blah blah, no more need to do Agility/Str/Int/+ to crit/ + to hit conversions to find out what % it will increase your actual output by, it gives it to you in an instant. And finally you should have some type of boss mod, I use Deadly Boss mods, there are many others and probably some others that are better out there at the moment, but I just haven't spent the time yet finding one.