It depends on what you are looking for. If you are looking to buy a home, most companies and many agents offer IDX searches which produce results directly from MLS listings. Some companies offer their company listings as first results in the search. All company and agent site searches are designed to bring your business to them.
there are sites that offer non-listed foreclosures. You have too look a little harder to get a full representation of for sale by owners.
A real estate professional can help you find what you are looking for, and provide or help you access thorough results. Please feel free to contact me if I can be of service.