From their site: Trulia is a real estate search engine that provides buyers with information about homes for sale, real estate trends and local market information. The Trulia API allows you to add data about regional real estate listings and Trulia search trends to your application.
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There are no source code libraries or how-to links for Trulia. If you know one why not add it?
here is another real estate search engine with over 1 million listings. www.viewmylisting.com
Trulia's mashups are pretty high quality. Can anyone say how good the API is comparatively? Maybe it's because they use Mashery. =)