I know I'm going to kick myself when you tell me but...
 

I'm drawing a blank on a mod I used to have that shows me if a mob or item is a quest item when I mouse over it. It showed up at the bottom of the tooltip eg.
Elder Brown Bear
0/4 Elder Bearskin
"Getchurskinon"
Given by Some NPC

If this sounds familiar can you help me out? It's one of those things I know I know but just can't reember it and I'll probably remember it a week from now hehe. Thanks in advance

LitheToolTip Doctor did this for me.

Fast Quest - http://wow.curse.com/downloads/details/3558/ ?

QuestFu and MobInfo do it as well I think.

LitheTooltipDoctor definitely doesn't do this.
It just moves the default tooltip.

Some of the other suggestions might be valid.
Personally the only mod I've found that does this consistently is MonkeyQuest.

One thing to note:
If you're using some questlog replacement, DoubleWide, QuestGuru, bEQL etc,
only get MonkeyQuest from the Monkey mods package not MonkeyQuestLog.
Extract in your addons folder but only keep MonkeyQuest and MonkeyLibrary.
Remove MonkeyQuestLog if you're using another questlog replacement to avoid conflicts.