aka Bookmarks
Legend:
Directories: Comprehensive lists.
Professional: Sites meant for professionals. Look for student memberships to gain full access to their archives.
Research: While Wikipedia is a good place to start if you are new to a given topic, you should consult at least 5 independent sources and follow at least one level of fan-out from those sources to say that you have done the initial round of research. Do not ignore age old ones either - Encyclopaedia Britannica.
Tests: Information related to a specific exam.
Tutorial: Getting started; step-by-step instructions to learn a specific concept.