SWStats>DmgMeters for E-peen.

Omen> KTM. I find KTM is buggy as hell even though
Especially when you use feign death, KTM doesnt wipe aggro on certain bosses. (it is not resisted)
BTW, Omen has a nice flash screen alert for aggro.

Witchhunt for simple spell alert. It makes hell lot of difference in pvp.

Bigwigs or Deadly Boss Mods based on personal preference. I prefer BigWigs because the warnings are more flashy and noticable.

BigTrouble is good for hunters for monitoring shot cooldowns if you are not using BM spec shot rotation (aka Steady shot/Autoshot macro spam)
MM or Surv needs to weave in multishot and arcane.

Prat for customizing chat frames and fonts.

Get Grid and Decursive if you are healer. Make spot healing 100% easier.
Customizing Grid for your own class can be a pain in an ass but its worth it. Check out Nihilum's website for an awesome grid customize settings for pallies.