I really love the service 'MyBuilder' gives in sifting out the ideal workman for whatever project you have, as they have a criterion for their workmen to adhere to which gives the customer confidence they are not going to have to potentially deal with a rogue trader.
The only thing I would suggest for their website is to allow the customer to upload photos before during and after the work has been done, as photos that workmen upload could be from anywhere, whereas when it's from the customer I believe it would give the review so much more validity and strength.