The Online Writing Lab at Purdue (OWL Purdue) - Excellent resource. In addition to their up-to-date versions of the most popular citation styles, they also host a number of helpful handouts dealing with writing and grammar.
APA Style Guide Online - Style and grammar guidelines with examples.
CSE (Council of Science Editors) - Citation and style often used with the natural sciences. This quick citation guide provided by PennState University Libraries.
CitationMachine, EasyBib, BibMe, etc. - These online tools can be helpful, but it's always best to check your citations against the official models available at OWL Purdue, or the stylebooks available in the library (ask at the Circulation Desk).
By installing a plugin into your browser and Microsoft Word, these Citation Managers can almost do all your citations for you. Be sure to ask for help if you have difficulties setting things up.