Plagiarisma is a free tool for teachers and students to use for detecting possible cases of plagiarism.  Works with text files, html, rtf, doc, docx, odt, and pdf.

Awesomescreenshot.com:  Annotated screen shots are very useful for giving directions on how to use a new web-based service or software.  This Google Chrome & Safari extension could prove to be very handy for making annotated screenshots.  (Firefox version coming soon.)