One of my favorite addons is Know-it-all!
It's a HoT and DoT tracker that is really helpful for PVP and PVE.
You can also set it to track internal cooldowns on trinkets and watch spells that it doesn't track by default. I find it works perfectly as is, after you put it where you want on the screen of course.
Here is an example of watching some crowd control effects:
This is extremely helpful in dungeons and raids if you are using CC and not just AOE'ing everything down. You can watch when the Mage's polymorph is ending and know that the tank will have to pick it up, or see when sap is ending so the mob can then be polymorph'ed for further CC.
For healers and casters this will help you increase your healing or dps done because you can watch your DoT's and HoT's on mobs you are no longer targeted on. It will also let you know when you need to refresh your spells on a target. For example, if the healer throws a HoT on the main tank but need to switch targets to heal someone else, the healer can watch the HoT on the main tank and refresh when it gets low.
Here is an example of HoT's on multiple targets:
As you can see, I am not even grouped and it is still tracking my heals. It also places a small "T" next to the bar of whoever you are currently targeting.
If you have any questions please feel free to ask!