Aywren's Sygnus.org Final Fantasy IV mega fan site...Art, fan art, Fan Fics, information, role-play, fan manga...just about the biggest fan-based domain for FF4 fans on the net. Definitely a must see. Many of the other webmasters below have contributed to Aywren's Fan Archive (including myself). Know what, see that link up there? You should have clicked it by now.

Doc's Final Fantasy IV Fan Site. Good info, great selection of Amano Art, lots of other great stuff check it out. Also be sure to also check out Doc's Fan Fiction "Final Fantasy White Light" for a good Final Fantasy IV reading adventure too! You can visit Doc's homesite at waterbearer.net

The "Not Serious!" website by Subaru-san, a person of Japanese origin, has a great Final Fantasy IV section. The link above goes to an English mirror of the FF4 website. This one is truly unique. Don't let the website's name fool you, Subaru-san has very serious information and treats Final Fantasy IV as a story and not as a game (Yes!) which means it has to be good ^_^. Subaru-san also makes use of the four NTT game books (which I'm sure are easer for Subaru to read) Check it out, I know you'll like it.

http://ffivonline.com
A great in depth coverage FF4 site run by Mathus. Lots of great content coming from many of the rare sources such as the NTT game books, not only that but a huge database of other great content. Definitely a great resource on the game. Make sure to stop by!

http://www.geocities.com/thedawezy/
Dawezy's Final Fantasy IV Network is one damn good FF4 site. He's got loads of great info and content on/about the game. There's also a lot of fan-based creations, like custom sprites and sprite theatre. Great FF4 site with a lot of personality and good humor.

http://www.fantasyanime.com/finalfantasy/ff4/
Alex's Final Fantasy IV section of his FantasyAnime.com domain has a lot to offer. Full size gifs of each world map, Bios on all the characters, Wallpapers, Screenshots, and much more. Another great stop in the online world of Final Fantasy IV! Check out his other interests at FantasyAnime.com.

SORRY! Melodic Solace seems to have disappeared on me...at least the mysidia.nu domain is "gone"! I haven't heard from Aa-Chan...so if anyone knows where she went please let me know!