World of Warcraft Spotlight
While this is just a small addon, it might be able to save your character's life. When your character's hp drops to a certain amount, the screenborders will turn red to warn you, so that you can save your character from death. Besides the red borders, both a sound and a text warning are available too.
Using the included commands, you can toggle text and sound warnings on or off and set a treshold that will trigger the health warning.
Simply clicking the "Install via Curse Client" button found on the main page is all you have to do to install this addon.
HealthWarning is a great choice if you tend to forget healing your character in time, it will warn you whenever their hp reaches a configurable treshold.