Visual Heal. Visual Heal is, obviously, a UI mod for healers -- so the rest of you can just go on about your regular schedule. What us healer-folk are going to be discussing will probably bore you to tears. Because, really, when you consider the mana efficiency of Greater Heal rank 4 over Greater Heal rank 7, my healing per mana increases by 5.9%, which allows me to....
...okay, now that we've run them off, we can talk. Just you and me, healer to healer. Visual Heal isn't what I'd call a must-have addon, but it does provide useful visual cues that I've found found helpful on more than one occasion. With Visual Heal installed, whenever you cast a heal, an additional health bar comes up near your cast bar showing your target's current health and your target's estimated health after the heal has been cast. As you can see in the image above, the green bar under my cast bar is Visual Heal. The green section is my current health -- full -- and the red portion is the amount the heal I'm casting will land for. (Color-coded red because it's all overhealing.) For me, it means instead of looking back and forth from my cast bar to my target's health, all I have to keep an eye on is Visual Heal, which is nice, since I've yet to be able to train my eyes to watch both bars at once.
While it's a nice toy to have in single-healer situations, it starts to shine in larger groups. If multiple healers in a group all have Visual Heal installed, the Visual Heal bar will show not only your heals on a target, but others' heals -- allowing for, potentially, much better cooperation on your healing team.
If you're interested in giving it a try, you can download Visual Heal from the WoW Ace Wiki, just scroll down to the bottom of their downloads page to pick up the latest version.