I used from ages a little addon:
achievecompare by strangelet.
It compares your achivement status when you click on a achivement link by someone else.
I think it is abandoned since last updated 11.01.2013 and curse client reports a not found home page here:
http://www.curse.com/addons/wow/achieve_compare
and wowinterface don't find it if you search.
I try to upload it here because I think it is a good and usefull idea and perhaps it finds someone that have the skill to updates it.
Cheers
