Useful addons

oRA2 - makes good tank windows, which atm i am broudcasting every raid
decursive - good for dispelling of any kind
omen - threat meter, very important bit of kit, you dont show up on it unless you have it (or at least thats how it used to be)
recount - handy for seeing hwo you died and checking out damage, but not essential, and spamming in raids generally considered a bad thing
pallypower - This addon saved LOADS of time back in BT sorting out paly buffs. even if you dont have a pally, you can use it to see who is buffing what, so you know who to ask if its fallen off
magic runes - awesome DK rune bar add-on, 1000000x better then the standard "bars"

im sure theres some healer specific ones, but ive never done that so

SmartBuff if u're not a pally but still have to buff a raid or yourself, to see who's missing one of your buffs.

ItemRack if u're dualspecced and need different sets of gear.

Altoholic if u have lots of alts, u can see which alt has new mail, how much money, what in their bank etc...

Tomorrows patch.

Equipment Manager

* When enabled from the Interface Options menu, this feature will allow players to store sets of equipment, easily swap between saved sets using hotkeys, and pull items directly from backpacks or bank slots (must be at the bank to equip inventory from the bank).