There's an old addon called Simple Mouseover's Target. (WoWUI link)

I'm wondering if someone could update, or recreate this if possible.

Take a look at Are You Lookin' At Me?

Thanks, this is close to what I'm after. Maybe I can talk the guy into fixing it up a bit more.

Curse needs a better search function for the addons. Mod authers should put a string of words that are descriptive of their addon, and curse can search the string, and not just the mod name.

I updated it, uploaded it to WoWI (I've stopped supporting WoWUI long, long ago).

http://www.wowinterface.com/downloads/info12361-SimpleMouseoverTarget.html

Cowtip also can tell you what is being targeted by the mouse over unit.