I have researched this very topic for days with very little luck...
I am assembling a “mixed breed identification blog” at
http://www.mutthut.blogspot.com. And I am inviting shelters, rescues, self-titled "experts" and dog owners to contribute photos and breed guesses. Please let me know if you plan to contribute so I can add a link to your organization (if applicable).
I decided to start up this site after acquiring a mixed breed pup a few days ago. I was disappointed to find so little for new owners or enthusiasts of mixed breeds. I found a few “what breed is this?” on message boards, but I thought the blog format would be easier to organize and search (especially with my limited computer knowledge!)
A little about me: I have worked in animal welfare for while living in the US (both in advocacy and in shelter work), and moved to the Caribbean a few years ago. I discontinued the professional side of animal rescue, and thought this would be a fun way to assist shelters and dog owners on a small level. This is a pet project (sorry about that) and I have no intentions to take anything from this but a good feeling and a chance to connect with other mutt-lovers
I truly hope folks will take the time to contribute with photos and/input. I think the Breed ID plays a large role in helping owners connect with their dogs and understand behaviors.