Ecco the Dolphin - Defender of the Future

Dark Sea
Game resources including level guides, video and music downloads, version information, and staff interviews.
http://www.ecco-darksea.com/

Ecco the Dolphin - Wikipedia
Very thorough description of the complete Ecco the Dolphin series with a great introduction and information about each of the games.
http://en.wikipedia.org/wiki/Ecco_the_Dolphin

Ecco the Dolphin: Caverns of Hope
An Ecco the Dolphin site covering a lot of topics with an ever-growing amount of information about the games and related stuff. News and Downloads can be found here, as well as a growing community.
http://web8.orcaserver.de/ecco/

Ecco the Dolphin Forum at GameTalk
A forum to talk with other ecco fans about anything like help and codes to even descussing problems.
http://www.gametalk.com/talk/pc/ecco_the_dolphin/page01.htm

Ecco:DoF guide
Half completed walkthrough for ecco:dof that has the original feature of being available both in English and French.
http://www.angelfire.com/games2/ecco/dcguide.html

Ecco-le-dauphin
A French Ecco the Dolphin site with a fan art work section and forum.
http://ecco-le-dauphin.ifrance.com/

GameFAQs: - Ecco: DoF Forum
Forum for ecco fans, also a collection of ecco resources including a FAQ.
http://www.gamefaqs.com/console/ps2/data/534932.html

Tim Follin's Burial Site
The creator of the music in Ecco the Dolphin: Defender of the Future presents excerpts from the music on his personal website.
http://www.timfollin.pwp.blueyonder.co.uk/

yivdesign
A website with many wallpapers designed by an ecco fan.
http://www.yivdesign.narod.ru/