Many university libraries review, collect, and organize material into subject or research guides.  Often these resources are published in electronic formats.  The University of California, Riverside's Infomine is a prominent example.

These guides highlight the most important sources in an academic discipline and usually provide links to online sites in a subject area.  The material selected for inclusion has been reviewed by librarians and is usually annotated.

Lovejoy Library has Research Guides available for a variety of subject areas.

Other excellent library subject directories include:  AcademicInfo.net, Librarians' Index to the Internet, and Penn State's Online Reference Sources.