is there a free site where you can list contractors that are or are not favorable

Joanne....that's actually a good idea. What type of contractor? General construction, plumbing or heating and air conditioning? I'd definitely look them up in Google Maps and leave a review there along with Yelp.

Do both because Google Maps is used for Droid Smart Phones and Apple Maps uses Yelp to source business reviews. You can also easily file with the BBB and ding their rating there. Those steps WILL help others avoid them.

If you're ever after an HVAC contractor, just Google "hvac dealer locator". There is a directory called Find an HVAC Dealer that lists quality contractors. They don't charge anyone, and only list quality companies. If you see a contractor on there that is bad, let them know and they'll yank them.