Here are some of my favorites links...
Links

BioDiesel
http://www.biodiesel.org
http://www.biodieselnow.com
http://www.biodieselamerica.org

GM / Chevrolet / Hummer:
http://www.gm-diesel.com
http://www.62-65-dieselpage.com
http://www.62-65-dieselpage.com/duramax6600.htm
http://dieselplace.com/forum/index.php
http://www.hummer.com

Dodge
http://www.dieseltruckresource.com
http://dodgeram.org
http://www.abhunter.com
http://www.turbodieselregister.com
http://www.dieselram.com

Ford
http://www.intellidog.com/dieselmann
http://www.powerstrokeregistry.com
http://www.thedieselstop.com
http://www.blueovalnews.com

Engines, Filters, Etc
http://www.cummins.com
http://www.lombardiniusa.com
http://www.fleetguard.com
http://www.baldwinfilters.com

Miscellaneous
http://www.bobistheoilguy.com
http://www.enjoythedrive.com

Mercedes
http://www.mbworld.org

Fuel Economy
http://www.greencars.com
CGR_LowTireResistance.pdf
http://list.priceweber.com/cummins/turbo_oct04/economy.html

Organizations
http://www.diesel.org
http://www.dieselforum.org
http://www.sema.org

RV & Towing
http://www.rv-links.com
http://www.rvhome.com
http://www.rvadvice.com
http://www.rvamerica.com
http://www.rvsafety.com
http://www.towrig.com

Volkswagen
http://www.tdiclub.com