Realtor.com -
This service lists properties nationwide. It complies the data from the various Multiple Listing Services across the country.
Zillow -
This is an independent company that compiles data nationwide from whatever source they deem creditable.
Trulia -
This site operates as Zillow does, collecting data nationwide from various sources.
Multiple Listing Service of Long Island -
This service lists all available properties throughout Queens, Nassau and Suffolk Counties. Most listings are linked directly to public to yield a more complete background of the property of interest. The Long Island Board of Realtors monitors the site for accuracy and compliance to State and Federal Regulations.