SpellDamage is a simple addon that provides a movable frame with the current spell damage. It was written to improve timing of active amplifiers for arcane mages, but it works for all classes.

The usage is quite simple, it is controlled via the slash dommand /sd or /spelldamage
/sd [show | hide | toggle] -- to show, hide or toggle the frame
/sd [school] -- to change the current school
/sd [font | size] -- to change the font or size
/sd [lock | reset] -- lock the frame in place or to reset some settings

The frame can be moved with shift-click & drag, or resized with alt-click & drag when it's unlocked. Resizing isn't that useful yet though.