Various helpful addons

Some useful addons for healing.

Big Wigs - not only for the healers, however it helps to switch the healing to certain targets, which are being for example sacrificed or taking some other extra damage.
Download here

sRaidFrames - allows to see all groups in raid, players in healing range, buffs and person with aggro and much more... :) , so you can send your heals to the right person faster
Download here

Clique - Click casting allows you to define the behavior the game takes when you click on a frame. This can be casting spells, running a macro, using an item, or something as simple as changing targets or assisting a unit.
Download here

Decursive - Helps to remove afflictions in a really easy way (all types: magic, poison, curse etc.)
Download here

Healbot - Helps healing in 5 to 10 man instances. Heals with single click on a player.
Download here

Raiding
A few addons that you should have installed if going raiding. There are others that provide the same functions, it's fine to use them too as long as they do the same :-)

Omen - Threat meter
Ora2 - oRA2 is a lightweight CTRA replacement