Crossref.org — find complete book and article citations by keywords, title, author, ISSN, ISBN, or DOI. Contains tools for generating citations in a variety of formats (APA, MLA, Chicago/Turabian, IEEE and more)
DOI.org — the Digital Object Identifier (DOI) system provides a technical and social infrastructure for the registration and use of persistent interoperable identifiers, called DOIs, for use on digital networks
EasyBib — free automatic bibliography and citation generator for MLA or APA format. The site includes style guides and a plagiarism checker as well.
Guide to Grammar and Writing — improve grammar, syntax, punctuation, sentence & paragraph structure and more with instructional quizzes & tutorials
IEEE Author Center — includes Author Tools for publishing and managing references in IEEE format