I believe the most popular healing addons, at least among the healers I know, are Grid and Clique.
Grid is a raid frame addon that's heavily customizable as to how you want your frames, buffs and debuffs to appear. Many druids use it to monitor who has rejuv on them, etc.
Clique lets you rebind your abilities to mouse buttons with modifiers, so right click might be Rejuv, and Shift right click might be Regrowth or whatever. It lets you just clique on raid frames instead of having to click and then mash a key.
Kathy, I said, "I'm lost" though I knew she was sleeping
I'm empty and aching and I don't know why
Counting the cars on the New Jersey Turnpike
They've all gone to look for America