Hi, does someone know whats the name of this addon is ?

http://www.gcguild.net/movie/yogg-saron-one-light-darkness

Its the bar under the castbar which shows you the remaining of your cooldowns

Ahh yes.  I used that addon during the beta.  It's called Ellipsis.  http://www.wowinterface.com/downloads/info10749-EllipsisMulti-TargetDoTTimers.html

A similar mod that I'd recommend is called Event Horizon.

There is also a scaled down version of Event Horizon called Coolline.  It is a great mod and very easy to set up.