Addon help.

[MakaveIi]

What is the addon that tells you when a ability if off cooldown?

[Arthroxin]

Fortexorcist, NeedToKnow, etc.

[ComputerNerd]

Depends on what sort of announcement you want.
Timers on the action buttons are usually from OmniCC.
Graphical announcements often from PowerAuras.
NeedToKnow or ForteXorcist for bars.
TellMeWhen or Doom Cooldown Pulse for icon based.

Those are some of the more commonly used ones as far as I can remember.