Shows last damage/heal done by spell as button's text.

There is 3 types of more or less accurate spell effect you can expect by casting:

Tooltip data from Blizzard (seems always lower than actual effect)
Heavy calculations and tuning like DrDamage do
Collect data and show (like this addon)

For now supports only Blizzard's action buttons and Bartender4 since v0.20.

How it works

Lot of spells quite plain: you pressing button, it have spell id, through combat log addon receives damage/healing info. For example you casting "Smite" - log contains spell cast started and damage info (effect will be shown on next spell cast).
Some spells more complicated: you casting instant spell "Icy Touch", it contains cast success, direct damage info, "Frost Fever" aura applied that causing extra damage per tick. Some of such spells solved already, some not (I have only several 80+ alts). So if you want to get more info read next section "Howto".

Howto

To add damage/heal caused by spell's side effects:

Find "Target Dummy"
Enable debug mode like "/le debug"
Cast your spell and see log in chat
There will be cast and auras info (format is "event, spellId, spellName, amount or buff"), add your side effect spellId to LE_SpellPartOf in Spells.lua like [sideEffectId] = mainSpellId
"/reload" and see is it works
Disable debug mode typing "/le debug" again

To toggle debug mode use slash command /le debug (or /LastEffect debug).