Non-Priests (or Priests that don't heal), you might just want to skip over this one. MendWatch is an AddOn that...well, the author explains it best:
MendWatch attempts to track the priest spell Prayer of Mending. A timer bar shows the current target of the Prayer of Mending spell with a countdown of how long the spell will last. When the target gets hit the amount the Prayer of Mending healed for will be shown and the bar will slowly fade away. If the Prayer of Mending jumps to a new target a new timer bar will be shown for the new target.
This AddOn fixes my one complaint about Prayer of Mending: it can be a bit hard to track who it's on or whether you even have one out there at all, when the battle starts getting thick. Just like Cloak of Shadows brings a Rogue vs. caster duel from "imba" to "insane," MendWatch brings PoM from "awesome" to "incredible."
If you're still reading but not familiar with Prayer of Mending, here's the spell's description:
390 mana, instant cast, 40 yard range
Places a spell on the target that heals them for 800 the next time they take damage. When the heal occurs, Prayer of Mending jumps to a raid member within 20 yards. Jumps up to 5 times and lasts 30 sec after each jump. This spell can only be placed on one target at a time.