This addon displays a large yellow number (and text for numbers > 60 seconds) on spell icons for action bars, bags, character sheet, and quest icons in the tracker. Font can be modified, and the cut-off for how long a cooldown needs to be in order to display on the button can also be changed. The number begins flashing when time remaining is less than 10 seconds.
This is an update for WoW >= 3.3.5 compatibility since 4.0's new API nuked some of the code here. The original addon was created by hshh and the compatibility has been fixed by TheCheat5 and Kiki.