www.zophar.net - A great site with lots of technical documentation and other information invaluable to emulator authors.

nesdev.parodius.com - The final frontier. This site has all the best NES technical info currently available. Also has a message board that is uncannily friendly to newbies.

www.pdroms.com - A site containing many legal ROMs for all systems, including the NES.

www.mightymo.8m.com - The web site of Mighty Mo, the official developer of NEStron's game genie database.