HealBot is a great addon for healers (duh...), but also for anyone that can buff otthers and cure diseases, poisons, curses and despell etc.

It gives you an icon for everyone in the group/raid, and just by clicking that icon you will automatically heal/buff/cure the target. You don't have to target any players, just click the icon. Icons changes color according to what action needs to be taken on the player. These colors can be customized.

You can also program what kind of buffs are most suited for every class, very handy for Paladins for example.

When out of combat, using the left mouse buttom will automatically choose the most logical spell on each player, so just by clicking it several times it will heal, buff, cure etc. just by left clicking, until there are no more actions necessary for that player. For healing it will also choose the most mana efficient heal to bring the player to full health.

When in combat you use different mouse-combinations to manually select the most appropriate action, like for example for me I use left buttion for Regrowth, middle for Rejuvenation and right for Healing Touch. Other functions can be programmed by using Shift/Ctrl/Alt while clicking, so you can have a lot of different spells programmed not even having to target anyone.

I've tried it out breifly.
It's pretty simple to install, and it sets itself up with some helpful default settings so you can literally have it up and running quite quickly.
A very good example of an application that does exactly what it sets out to do, and does it well.

However, I should point out it's pretty much designed completely around the mouse.
If like me you prefer to heal using keyboard shortcuts this won't help.