Try Collarity. It claims:
The Collarity Compass enables publishers to deliver more effective site and Web search results by providing each visitor with the power to automatically define what is most relevant for them and provide search guidance for future searchers. Every visitor on a publisher’s site becomes both an information provider and an information consumer. Every user’s searches accrue to the benefit of every other user. As site visitors search, the system automatically and anonymously gathers, organizes and shares search knowledge.