This widget will display the last damage you did, and it can add consecutive damage numbers together like damage batching in TF2. You can change the appearance of damage numbers, the time that it's displayed for, and the window for batching damage. It works correctly when spectating (it tracks damage for all players simultaneously and will show only damage for the currently spectated player). It does not know the difference between victims though, so in FFA or TDM it will show cumulative damage for all victims, not just one. The other short coming is that it doesn't know where the victim is, so numbers appear on a fixed spot on the screen.
The download can be found here. You'll need KeredDamageNumbers.lua and the entire Utils and OptionsGadgetse folders.
If anyone has feature requests for this I'll see about adding them.