http://wow.curse.com/downloads/wow-addons/details/nvgearscore.aspx

It's called gear score.  If the person is close enough to inspect, you can mouse over them and the tooltip that shows their name-guild-location will also be populated by a number.  You can rate a pug very quickly by just mousing over him instead of having to look up his gear on the Armory.

Sapience is rolling around 4500, Nethrill is rolling at about 4200 and I'm rolling at just over 3900.  That will give you an idea of where gear score will place people with that kind of gear.  Someone in greens and blues will be below 3k.  Very handy.

And if I'm spamming your forums it's because I'm not real fond of my job and this is new and interesting.  Please feel free to tell me to cease and desist at any time.