Technical Writing and English Usage
The AIP Style Manual, 4th ed. (THE authority for physics papers)
Chart of Proofreader's Marks (from AIP)
Oxford English Dictionary ("the definitive record of the English language" according to them, but note that US English spelling and usage may differ—we colonials have our own rules)
George L. Trigg's Advice for PRL Authors, Phys. Rev. Lett. 42, 747 (1979). [Bragging rights if you can explain why Professor Trigg's #12 is incorrect.]
Not available online, but highly recommended for your library if you're serious about writing well:
The Elements of Style, William Strunk, Jr. and E.B. White, 4th ed. (New York, Longman, 2000).
Garner's Modern American Usage, Bryan A. Garner (Oxford, Oxford University Press, 2003).
Authoritative, comprehensive, and funny. Ms. Particular agrees with Garner about 98 percent of the time, but he is
The Careful Writer, Theodore Bernstein (New York, Atheneum, 1965). Old but timeless.
The Synonym Finder, J.J. Rodale (Emmaus, Pennsylvania, Rodale Press, 1978). Some synonyms provided are too colloquial for scientific writing, but a good spur when you get stuck for exactly the right word.
TeX and LaTeX Resources
LaTeX Project (includes a searchable "bugs" database)
Comprehensive TeX Archive Network (provides the most complete and up-to-date TeX-related software)
Visual Display of Quantitative Data
"Introduction to Refereeing" (from the Institute of Physics)
Comparison of reference management software systems
Citing Sources and Avoiding Plagiarism
The Plagiarism Spectrum—comprehensive resource that defines the various degrees of plagiarism to help you avoid inadvertent plagiarism
The Plagiarism Checker—free site to detect plagiarized text
Introduction to Responsible Conduct of Research for Physics Students—The Department of Physics' statement of its commitment to research integrity and ethical guidelines for beginning researchers