Purdue University's Online Writing Lab (OWL) is a well-known source of citation and writing information. Everything from starting the writing process to MLA/APA/Chicago formatting and style guides.
The Chicago Manual of Style Online is completely searchable and easy to use, providing quick answers to your style and editing questions. The Q&A content is fully searchable along with the content of The Chicago Manual of Style. The Chicago Manual of Style Online also provides convenient Tools, such as sample forms, letters, and style sheets.