1. Reference Extract
"Reference Extract is a targeted web search engine. It is built from the expertise of AskA services geared to the education audience. The Virtual Reference Desk team has identified high-quality archives of FAQ's and previously asked questions. These sites were then indexed, and the result is an easy to use, quality oriented search engine: Reference Extract."The concept is interesting but doesn't have much content yet.
There's a blog for its development if your interested.
"Harvest all the URL's cited in reference answers and feed them into a search engine. The idea is not to build a general purpose search engine...like Google ...but to build a smaller search engine of "reference approved" sites and sources."David Lankes has a lecture (powepoint/podcast) on development of these products and their concept of 'reference authoring'--this process of using digital reference output to populate a focused search engine. (By the way, being able to view PowerPoints synced with the audio is pretty cool.)
Gets URLs and builds database of sites based on frequently cited URLs. Then spidering/crawling those URLs.