I have used ServiceMagic (http://www.servicemagic.com/) in the past and been pretty happy with the results (I was able to find a good electrician through it), but that was along with checking up on the contractor through a Google search. I wouldn't just trust a ServiceMagic reference without being able to find some other proof of the quality of the contractor. I do not have a specific list but usually if the contractor has a quality website, business license, 800 phone number, etc. then they get some points toward being awarded my business.