Still more reference link collections:
www.intute.ac.uk: Intute is an annotated collection of web resources for education and research, created by a network of UK universities. Subject specialists select and evaluate the websites and write detailed descriptions. This site contains over 120,000 resources in the arts and humanities, health and life sciences, social sciences, and science, engineering, and technology. You can browse through the “Subject A-Z” list of hundreds of specific subjects, or use the search functions. The advanced search function will allow you to limit your results by the type of resource.
www.nytimes.com/navigator/: The New York Times Newsroom Navigator is a collection of web resources used by journalists and editors at the New York Times “as a starting point for their forays into the Web… without forcing all of them to spend time wandering around to find a useful set of links on their own.” In addition to the Newsroom Navigator, there are separate Business Navigator, Politics Navigator, and Health Navigator link collections.
Update January 2015: Intute, sadly, has disappeared from the web. For alternatives, see my Sources page.