I'm a big fan of Zillow.com. Since their initial site debut, they have consistently challenged the other "players" in the Web Real Estate landscape by providing innovative features and offerings. For users, they offer a mindblowingly good "wiki", an active "discussions" area (a Q&A forum style place to get free professional opinions), and an intuitive, robust property search and display interface. For Real Estate Professionals, Zillow offers excellent co-branding, property marketing, and networking opportunities - all for free. There are paid advertising options. According to statistics from Hitwise, Zillow.com is consistently among the 10 most visited real estate sites on the web -- and for good reason.