"The Online Writing Lab at Purdue University houses writing resources and instructional material, and we provide these as a free service of the Writing Lab at Purdue. Students, members of the community, and users worldwide will find information to assist with many writing projects." The following resources directly link to the Purdue OWL guides for common citation styles.
From the Online Writing Lab (OWL) at Purdue University, a great, concise resource for a lot of basic citations according to Chicago Manual of Style (CMS).
MLA Handbook, 9th ed.
The Chicago Manual of Style