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
Tips from me (Adam Turner) on using text from other papers in engineering and sciences.
A good collection of links on avoing plagiarism from UCLA
Video presentation on using references and avoiding plagiarism (general academic writing).
Format of references for all fields, designed for undergraduates
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