"The Online Writing Lab (OWL) at Purdue University houses writing resources and instructional material, and we provide these as a free service of the Writing Lab at Purdue. Students, members of the community, and users worldwide will find information to assist with many writing projects. Teachers and trainers may use this material for in-class and out-of-class instruction."
Purdue OWL has a section on plagiarism that includes an overview, best practices, and exercises in determining what plagiarism is.
Turnitin.com provides a quiz with which you can test your knowledge of plagiarism. It takes approximately 3 to 5 minutes to complete. Click this link to access the quiz.
Yale's Center for Teaching and Learning has a detailed guide to understanding plagiarism and citation. Included on the site is another brief quiz.