Citation Web Guides

Acton-Boxborough Regional High School Guide to Writing and Research (MLA based): http://ab.mec.edu/abrhs/pdffiles/Guide-Writing-Research.pdf

APA: (American Psychological Association) used in the sciences & social sciences
Purdue OWL: APA Guide: http://owl.english.purdue.edu/owl/resource/560/1/
APA Style: http://www.apastyle.org/

MLA: (Modern Language Association) used in literature & other humanities
Purdue OWL: MLA Guide: http://owl.english.purdue.edu/owl/resource/747/1/

Chicago: Author-date Style: used in social sciences
Chicago Manual of Style Quick Guide (click on author-date tab): http://www.chicagomanualofstyle.org/tools_citationguide.html

Chicago: Notes-bibliography style: used in arts, literature, history
Purdue OWL: Chicago Manual of Style Guide: http://owl.english.purdue.edu/owl/resource/717/01/
Chicago Manual of Style Quick Guide (notes-bibliography is the default tab): http://www.chicagomanualofstyle.org/tools_citationguide.html

Turabian: (a simplified Chicago style) used in humanities, social & natural sciences
https://www.library.georgetown.edu/tutorials/research-guides/turabian-footnote-guide

Research and Documentation Online 5th Edition by Diana Hacker and Barbara Fister
covers MLA, APA, Chicago and the CSE (Council of Science Editors)
http://bcs.bedfordstmartins.com/resdoc5e/

Online Citation Generators

CiteThisForMe : "Automatically create your bibliographies, citations, and works cited lists in the correct format using the APA, MLA, Chicago, Vancouver, or Harvard referencing styles - instantly!"

EasyBib : "Free Automatic Bibliography and Citation Maker. Save time by creating a Works Cited page instantly in MLA, APA, or Chicago!"

Plagtracker's Citation Generator :  Citation gererator helps you cite web resources, books, and periodicals in APA, MLA, or Chicago style.