Mash-ups have become exceedingly popular these days especially after Google placed its API (application programming interface) or “hooks” in the public domain to let innovative companies combine different sources of data to a create new effect. One of the best uses of the mash-up concept in real estate to date has been created by Trulia.com . Trulia was created by Pete Flint and Sami Inkinen in the summer of 2004 while they were graduate students at Stanford University. Note: the Google founders also went to graduate school at Stanford. I had the pleasure of speaking with Sami at length at Brad Inman’s Real Estate Connect in New York this month.
Here’s some more information about the service and their philosophies. 
Trulia is essentially a vertical application of a Google search.
I heard about Trulia last fall through word of mouth and have followed their popularity in California. I added a post about Trulia [Matrix]  a few months ago. The concept was straightforward and the site seemed to place tremendous emphasis on simplicity. Their data feeds are from public web sites, not MLS systems since that information is proprietary.
New York seemed to be ripe for this type of service as a compliment to what already exists in the public domain because it culls together a variety of information into one web page. When Trulia decided to launch in New York, they came to my firm Miller Samuel  as well as Property Shark  to provide additional content for users. The result of this mash-up is a lot of data useful to potential homebuyers interspersed within the listing information being searched.
Trulia is not a real estate broker and in fact, has sought out cooperation with the brokerage community. They have positioned themselves as a way for brokers to leverage the exposure of the listings already placed out on the web, and not as competition. They make their money from online advertising.
Among my favorite features are being able to create an RSS feed so the user can see new listings that meet their search criteria as the become available. I also like being able to save custom searches and their listing stats are particularly useful. Rarely do new web service sites come along that I get excited about.