"Grid is a modular, lightweight, and screen-estate saving grid of party/raid unit frames."
A very good raid frame which is very useful for healers. (Clbull)
Note: it can take a lot of fiddling to set up how you like it, but lots of tutorials exist for it and once you get the hang of it, it's easy. (lilserf)
"Clique is a simple GUI that lets you assign click-casting for any number of unit frames. 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."
Excellent when used with Grid to let you bind various clicks (right, left, middle, shift-left, etc) to different spells. (lilserf)
"Decursive is a cleansing mod intended to render affliction removal easy, effective and fun for all the classes having this ability."
I use Decursive, and I believe it (or a similar addon) is not only helpful, but imperative. (Hhoky)
"VuhDo is a raid monitor similar to CTRaidAssist or Blizzards built-in raid frames. Basically this is about displaying the health of raid members in form of clearly arranged bars. VuhDo is primarily directed to healing classes, but will make use to almost any other class. Moreover several healing spells or other actions can be asserted to mouse clicks on those bars (Click-Heal)."
This is a solid replacement for Grid, Clique, Healbot, what-have-you. Full raid frames, ability to bind spells (and Macros!) to key and mouse clicks on those frames, integration with the incoming heals libraries so you can tell when someone is going to be fine or not, ability to display HoTs easily, and best of all -- it works out of the box. Grid can take some time to set up properly, but VuhDo just works. (Celairia)
Player versus Player
"Gladius adds enemy unit frames to arenas for easier targeting and focusing. It is highly configurable and you can disable most features of this addon."
Gladius is every arena player's dream. Just like VuhDo it will run out of the box; you can simply drag the window to where you'd like and enter an arena. However, if customisation is your forté then there will be plenty to keep you busy. In addition to the usual Blizzard UI features of tracking health/mana counts and spellcasting, (albeit in a prettier way,) Gladius tracks enemy trinket up/downtime and drinking/first-aiding, as well as several important buffs and cooldowns. (Roble)
"VisualHeal is a small and simple but powerful tool for all classes that visually shows your heals to others and heals incoming to you from others by means of two information-packed, yet intuitive bars."
It shows all incoming heals to my target (great for watching overheals, or interrupting heals to maximize raid healing). (Hhoky)
"SmartRes puts bars on your screen that have timers for the various resurection spells: Resurrection, Ancestral Spirit, Revive, Redemption, and tells you who is casting on whom, how long it will take to bring the target back to life."
I also use an incoming Rez addon, called SmartRes, but that's just my personal preference, to maximize rez's. (Hhoky)
"A mod that tracks who Prayer of Mending is jumping to and how many charges you have left. Displaying it in a small movable window frame." (Hhoky)