APA, MLA and Chicago
Diana Hacker's Online Guide to Research
http://www.bedfordstmartins.com/hacker/resdoc

MLA
A Practical Guide to MLA Documentation
http://www.ccc.commnet.edu/mla

APA
A Guide for Writing Papers Using APA Style
http://webster.commnet.edu/apa/apa_index.htm

Help with Citing Web Sources in APA Style
(Web Extension to American Psychological Style)
http://www.beadsland.com/weapas

Sociology
coming soon

CBE (Council of Biological Editors):
coming soon

Chicago
http://www.lib.ohio-state.edu/guides/chicagogd.html
http://www.bedfordstmartins.com/hacker/resdoc/history/sample.htm

Columbia Online Style
http://www.columbia.edu/cu/cup/cgos/idx_basic.html

Nursing
http://healthlinks.washington.edu/hsl/styleguides/nlm.html