I've compiled a list of 10 web sites which, I think, can be useful and informative when researching John Donne and his works.
1. Luminarium - Contains easy to read sections on his quotes. works, life, and more.
2. John Donne @ Poets.org
3. http://www.online-literature.com/donne/ - Various articles and essays on John Donne
4. http://www.englishverse.com/poets/donne_john - Short introduction to John Donne
5. Searchable database of John Donne's sermons
6. John Donne @ Wikiquote
7. http://www.sonnets.org/donne.htm - A large selection of John Donne's works
8. Poetseers.org - A large selection of John Donne's poems
9. A biography of John Donne at Britainexpress.com
10. And finally, last but definitely not least, our very own Crossref-it.info. Get John Donne's biography, poem analysis, context material and timeline - all in one place (also contains resources on other Metaphysical poets)