APA Style.org: Information on citing electronic references in APA style.
Duke Libraries Citation Site: Instead of directing you to rules for a particular style guide, this site lists examples of four citation styles (MLA, APA, Chicago Style, and CBE) for any type of source: book titles, articles, newspapers, web sites, etc. This is a helpful site for anyone who wants to see the differences among styles.
The MLA Style Center: The MLA Style Center offers resources published by the Modern Language Association to help writers. This site includes a page with information about updates in the 8th edition of the MLA Handbook, as well as a quick guide to creating a Works Cited page and a guide for setting up a research paper.
APA Style CENTRAL: APA Style CENTRAL is published by the American Psychological Association and offers a number of resources for applying and using APA Style. The site has quick guides, samples, tutorials, and quizzes to help writers.
Chicago-Style Citation Quick Guide: The Chicago-Style Citation Quick Guide is published by the University of Chicago Press and is an official accompaniment to The Chicago Manual of Style. This quick guide offers writers sample citations for various types of sources, demonstrating both proper notes and proper bibliography entries for sources.
Purdue Online Writing Lab: The online writing center at Purdue University has an amazing library of 100 handouts on common writing problems.