Good guides to the different kinds of citations in each field: APA style is standard in Linguistics and many other social sciences, but MLA style is more common in English Literature.
Reporting Verbs (describing the research of others)
Excellent guide to avoiding plagiarism and references. It is especially good on problems with paraphrasing in science, which is often not included or treated too simplistically in other materials.
PDF version of the site
A good collection of links on avoing plagiarism from UCLA
Video presentation on using references and avoiding plagiarism (general academic writing).
Useful link on reporting verbs (verbs used to refer to other sources) but better for social sciences and not as useful for sciences.
How to use sentences from other papers and rewrite them in your own words.
Tools to check for plagiarism