FanFiction.net - Possibly the largest fan fiction archive on the Internet. A great place to read, post, and study fan fiction.
Fan Fiction Electronic Pathfinder. The Writers U. site I've mentioned in the other pages.
Frequently asked questions about fan fiction.
Dr. Merlin's Guide to Fan Fiction. An amusing and practical look at fan fiction.
The (Original) Mary Sue Litmus Test. Is your character a Mary Sue? Find out here.
Self-Insertion and Mary Sue-ism. A fascinating look at Mary Sue's place in fan fiction.
How to Write Marginally Readable Fan Fiction. Similiar to Dr. Merlin's site but delivered in a snarkier tone and including many sections about grammar. Highly recommended.
Fan Fiction Cheat Sheet. Links to sites to help improve your writing with the little things. The sites include things like Medical Procedure, British Slang dictionaries, and Police Jargon.