http://www.angieslist.com/

they are offering a full year membership free of charge if you sign up within the next few days. normally it is ~$6 a month. the list allows you to search for various contractors, garderners, lawnscapers, roofers, electricians, etc. and see the reviews from other consumers.

 

looks like just major cities

 

Yeah, their reach is limited, but one of my neighbors swears by it. Personally we do our own homework and usually go through recommendations and so on, which is pretty much what that site is all about... so we stayed away from it... but I have heard good things about it.