Priests - (or healers in general)
Clique - While not class specific, it is a huge part of my daily play allowing me to click cast heal or decurse an entire raid if needs be.
Dorje Healing Bars - Very useful as a priest and a healer in general, allows raid and group members to see who else is healing your target and yourself. This allows for a much more efficent method of healing to avoid mana wasting overheals.
Heal Organizer - As a raid healer lead I need to always assign various classes to certain people i.e. MT's etc. and this gives a GUI method which is easily broadcast to the desired channel.
Sacred Buff - Great alternative until Serenity is back up and running again. Both give a nice, neat and compact access to various buffs, debuffs, mana and health as well as reagent counts.