Most have been mentioned but I wanted to suggest doom pulse cooldown. Its a addon that fades a tooltip in and out of the middle of your screen when your cooldowns come off cooldown.
With healers, I want to advise them to be careful of addons like healbot / xperl(i think this one is similar to healbot, I didn't bother I thought it was ugly and distracting). Some can use them, myself and others cannot.
There are several reasons why for this. One is that it does not hold targeting. Another is that it ties your mouse up. This is because you have to hover over a particular unitframe to heal(its like a form mouseover healing). This consequently makes moving difficult and can also make you tunnel on your frames(decreases field of vision since you constantly have to babysit your frames). Some people can manage because they've
already gotten used to it. I think that Healbot is great for pve but not pvp.
If you want a healer raid frame addon I heard that grid is very good.
I think that the mandatory ones for everyone are:
Battlegroundtargets (Shows enemy team bars, names, class, etc etc in a small box), also has focus targeting and targeting functions)
Capping (Displays both fc's hp and flashes red when actively taking damage)
Icicle (Displays enemy players used cooldowns over the cntrl v player bars. ESSENTIAL for juking)
Some form of cooldown tracker. I use doom pulse cooldown.
And MAYBE gladiator losa(verbally calls when spells and cooldowns are used). It seems like a heavy addon for me so I recommend having a good computer. I stopped using it due to errors on my desktop and for choking my laptop to death with lag.
/endrant