There are several players in this space. Perhaps the largest are HomeAdvisor (formerly ServiceMagic) and Angie's List.

There are some distinguishing factors: one of the biggest is that we use a heavily data driven approach. One of our goals is to consider every single licensed contractor in the United States. In most areas, the actual number of contractors who are licensed to practice is an order of magnitude (or two) higher than what is considered by most sites. We actually just look at the actual population of contractors to help a homeowner determine the right choice.