Noah's Classifieds is a popular free classifieds system. A commercial version is available from the website.
From the Author
Noah's Classifieds is the Internets immensely and most popular Classifieds WebSite-Ware on the Web. We now have a software series with the most modern Web-2.0 feel, features, and amazing new feature variations and functionality and we have a truly created on on going and ever growing recipe for a great Classifieds Ads WebSite Software that is perfect for any project.
Authors website: http://www.noahsclassifieds.org/