CombatReplay addon records boss' fights and you can reproduce them after! It has many many cool information. Take a look at the video below.
How it works?
1. During combat it doesn't do any calculation, it just saves all info. => CPU friendly, uses less CPU than any other addon. But uses a lot of RAM memory at the moment, nowadays it's not a problem, it's just a number you will not have any lag ;)
2. When you decide to see a replay it reads all saved information and does all necessaries calculations, finally here it uses your CPU (when you don't care)
/cr or /replay to open it.
Important tip: if you put a mark on the tank it will be easy to know where the boss is at the moment. Because its too difficult to add the boss position and I have no free time (and no stimulation without donations) for this.
test example (the old one):
http:www.youtube.com/watch?v=Ac-Mm3LpoA4
more about addon: http://www.phoenixstyle.com/CombatReplay.php