In no particular order:

http://peachparts.com/shopforum/forumdisplay.php?f=15 Probably busiest MB Diesel forum out there, organized by fuel type ALA MercedesShop.
http://www.benzworld.org/forums/w123-e-c...-te-class/ 2nd busiest but organized by chassis
http://schumanautomotive.com/forums/foru...y.php?f=19 Super Turbo Forum
http://dieselgiant.com/mercedes_diesel_m...e_tips.htm Good list of maintenance items, great videos
http://www.dieselbenz.info/wiki/index.ph...=Main_Page Diesel benz wiki. Needs work but there are some gems in there
http://mersuforum.net/forum/viewforum.php?f=29 The Finnish board, Not much activity in the english forums but lots of easter eggs if you know Finnish.

Parts:

http://www.autohausaz.com/ Free shipping on $50 or more. Generally best price.
http://www.buymbparts.com/ Free shipping on $75 or more. Great resource for off the wall items like 15mm sway bars.
http://catalog.mercedesshop.com/ Competitive on some parts, also stocks used OEM
http://pullapart.com/ Easily best website for a junk yard I have ever seen. They DO NOT distinguish between 300D or 300SD.

http://www.junkyarding.com forum where you can find local u-pull-it style junkyards, still getting started, but gaining momentum quickly.

Paul

http://www.car-part.com/- I've found some OM606's with this site...never bought anything as a result of searching though.

Beers,

Matt

Boostnbenz's website

http://boostnbenz.1baddsm.com

Cached here as it tends to disappear when you need it.

http://superturbodiesel.com/benz/boostnb...m.com/DIY/

http://www.kastdiesel.com

My Fuel Injection store. I offer discounts to members of this site.

I edited a few posts to correct link errors caused by the software change.

Here are some more useful links.

Russian VIN decoder. VERY useful!
List of nearly every turbo ever used on a car or truck.
Turbo calculator. Helpful in calculating CFM requirements for different RPM, boost psi and altitude settings.
Online parts catalog.
Mercedes Diesel news website.
http://www.coloradobiodiesel.com/
List of every Diesel engine Mercedes' has used in a passenger car around the world from 1986 to 2004.
Conversion parts to put a V8 in almost anything.

partsgeek.com

rockauto.com (sometimes good prices, usually lower quality stuff for MB)

http://www.4btswaps.com small contingent of MB users, but they have enough varied other brasnd that there's alot to glean.

Ed

No, sorry. They're run by spoiled brats with no actual mechanical knowledge..