Show possible reputation gains in tooltip
 

Looking for an addon that will show the amount of possible reputation you will earn if you kill targeted mob (and obviously show which faction the rep will be for)

For example, I mouseover a mob and the tooltip would include the text "+10 reputation with Frostwolf Clan" or whatever faction/amount it rewards.

This sounds like what Overachiever does with its tooltips for achievements. I think your best bet here is to contact the author of Factionizer to see if they can add a tooltip feature to the addon. If that can be done I think this is a great idea.

http://www.wowinterface.com/downloads/info20490-Factionizer.html

I will do that but I would much rather it be a separate addon, with just that function. I don't need all of the features that Factionizer, FactionFriend and the like all provide. I thought maybe it would be something that someone could provide code for and I can make the addon myself. But I am no author/developer so maybe it requires more work that I originally thought.