I think this may be the one you're thinking of:
http://www.charitywatch.org/
Well, after I posted that Charity Watch link, I was looking at their site in more depth and am not sure it's the one I was remembering! I found another page with links to SEVERAL such sites that help evaluate charities here:
Effective Donating To Help You Give Wisely
http://www.heartsandminds.org/links/givelinks.htm