Chicago Manual of Style Frequently (and not so frequently) Asked Questions
Answers to some of the most common questions about using Chicago style.
http://www.press.uchicago.edu/Misc/Chicago/cmosfaq/cmosfaq.html

MLA Style
A brief guide to the style from the Modern Language Association.  See frequently asked questions, a guide for electronic citations, and a description of the MLA Handbook and MLA Style Manual.  From the MLA Homepage, click on the "MLA Style" link.
http://www.mla.org/

Online: Citation Styles Index
From Bedford St. Martin's, this guide will tell you how to cite WWW sites, email messages, web discussions, listserv messages, newsgroup messages, real time communications, FTP, gopher, and telnet sites in MLA, APA, Chicago, CBE styles. 
http://www.bedfordstmartins.com/online/citex.html

Online Writing Lab (OWL):  Resources for Documenting Sources
From Purdue University, this comprehensive guide describes all aspects of citation style for print and electronic resources and contains useful handouts.
http://owl.english.purdue.edu/handouts/research/r_docsources.html
For the OWL handout on APA style:  
http://owl.english.purdue.edu/handouts/research/r_apa.html
For the OWL handout on MLA style:
http://owl.english.purdue.edu/handouts/research/r_mla.html
For the OWL handout on citing electronic resources:
http://owl.english.purdue.edu/handouts/research/r_docelectric.html