Wikiosity is a project, a web service, and a concept : find the wikipedia articles (or *wiki articles for some wiki collection) most closely related to a small set of topics you care about, and present them together as background reading.
The algorithm does some checking of second-order links, filters out some common types of links, and does some other nifty things to keep its selection of related articles topical and useful, and correlated with the quality of the articles as well as the closeness of the subjects.
 Web service
For an online example of the code in action, see
http://wikiosity.com - Site inactive as of 06/14/2009, which should redirect properly.