idTip (by silverwind)http://www.wowinterface.com/downloads/fileinfo.php?id=17033#info

core.lua

local select, UnitBuff, UnitDebuff, UnitAura, tonumber, strfind, hooksecurefunc =
	select, UnitBuff, UnitDebuff, UnitAura, tonumber, strfind, hooksecurefunc

local function addLine(self,id,isItem)
	if isItem then
		self:AddDoubleLine("ItemID:","|cffffffff"..id)
	else
		self:AddDoubleLine("SpellID:","|cffffffff"..id)
	end
	self:Show()
end

-- Spell Hooks ----------------------------------------------------------------
hooksecurefunc(GameTooltip, "SetUnitBuff", function(self,...)
	local id = select(11,UnitBuff(...))
	if id then addLine(self,id) end
end)

hooksecurefunc(GameTooltip, "SetUnitDebuff", function(self,...)
	local id = select(11,UnitDebuff(...))
	if id then addLine(self,id) end
end)

hooksecurefunc(GameTooltip, "SetUnitAura", function(self,...)
	local id = select(11,UnitAura(...))
	if id then addLine(self,id) end
end)

GameTooltip:HookScript("OnTooltipSetSpell", function(self)
	local id = select(3,self:GetSpell())
	if id then addLine(self,id) end
end)

-- Item Hooks -----------------------------------------------------------------
hooksecurefunc("SetItemRef", function(link, ...)
	local id = tonumber(link:match("spell:(%d+)"))
	if id then addLine(ItemRefTooltip,id) end
end)

local function attachItemTooltip(self)
	local link = select(2,self:GetItem())
	if not link then return end
	local id = select(3,strfind(link, "^|%x+|Hitem:(%-?%d+):(%d+):(%d+):(%d+):(%d+):(%d+):(%-?%d+):(%-?%d+)"))
	if id then addLine(self,id,true) end
end

GameTooltip:HookScript("OnTooltipSetItem", attachItemTooltip)
ItemRefTooltip:HookScript("OnTooltipSetItem", attachItemTooltip)
ItemRefShoppingTooltip1:HookScript("OnTooltipSetItem", attachItemTooltip)
ItemRefShoppingTooltip2:HookScript("OnTooltipSetItem", attachItemTooltip)
ItemRefShoppingTooltip3:HookScript("OnTooltipSetItem", attachItemTooltip)
ShoppingTooltip1:HookScript("OnTooltipSetItem", attachItemTooltip)
ShoppingTooltip2:HookScript("OnTooltipSetItem", attachItemTooltip)
ShoppingTooltip3:HookScript("OnTooltipSetItem", attachItemTooltip)