Plagiarism
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.
http://ori.dhhs.gov/education/products/plagiarism/
PDF version of the site
http://ori.dhhs.gov/education/products/plagiarism/plagiarism.pdf
Tips from me (Adam Turner) on using text from other papers in engineering and sciences.
http://www.hanyangowl.org/media/references/usingreferencesinthesciences.pdf
A good collection of links on avoing plagiarism from UCLA
http://gsrc.ucla.edu/gwc/resources/citation-practices.html
Video presentation on using references and avoiding plagiarism (general academic writing).
http://www.unc.edu/depts/wcweb/esl/videos.html
Using sources
Format of references for all fields, designed for undergraduates
http://www.dianahacker.com/resdoc/
Reporting verbs
Useful link on reporting verbs (verbs used to refer to other sources) but better for social sciences and not as useful for sciences.
http://www.writing.utoronto.ca/advice/english-as-a-second-language/referring-to-sources
Paraphrasing
How to use sentences from other papers and rewrite them in your own words.
http://www.monash.edu.au/lls/llonline/writing/information-technology/sources/2.5.3.xml
Tools to check for plagiarism