Understanding & Preventing Plagiarism: Strategies & Resources for Students and Teachers is a colourful and well-designed page with advice, as the title suggests, both for learners and for educators.
The Center for Academic Integrity (CAI) is affiliated to the Kenan Institute for Ethics at Duke University. It is a consortium of over 320 institutions in North America which aims to provide a forum to identify and affirm the values of academic integrity. It has a useful link to the honour codes of its member universities.
This is a 'plagiarism-risk quiz' (using passages from an economics text) developed by the Goucher College Writing Program. It is an interactive resource that students might find helpful.