Very comprehensive list from the University of Arizona. Links to online citation formats for subjects such as medicine, legal writing, music, biology, other sciences, etc.
Brief Guide to Citing Government Publications - A well organized site by the Government Publications Department of the Regional Depository Library at the University of Memphis (TN).
Citing Electronic Information in History - An extremely large, comprehensive site with huge amounts of textual background. Good bibliography.
CSE Citation Style from the University of Wisconsin-Madison Writing Center. Scientific Style and Format: the CSE Manual for Authors, Editors, and Publishers, 7th ed. Council of Science Editors, 2006. [Index and Table of Contents are available at the CSE Website]
A Guide to Citing Maps - from Ohio Wesleyan University
ACS Style Guide -- from University of Wisconsin, Madison