For those 6.3/6.9 fans out there check out http://www.m-100.org
houses the archives for some very active email lists (although the site crashed recently, so the archives may not be up for some time yet). You also can join the appropriate list there - they have around a dozen different ones.
has forums like Mercedesshop - just not quite so good IMHO. They are organized by chassis though, so it may be easier to find info related to your car.
A basic info site for 109/109 owners or want to be owners is http://www.geocities.com/SiliconValley/Pines/6633/w108w109/w108owners.html
. Geocites has some sort of lame traffic limit, so it's unavailable if too many people hit it in an hour.
A 126 fan site is available at http://home.concepts.nl/~vlimmere/index2.HTM
, and finally, let's not forget the guys over at Diesel discussion, who put together http://mbdieseldiy.tripod.com/index.htm
which has some of the more common diesel jobs. Its aimed at the 123 folk out there.
Oh, and how can I forget the 107 SL site. Quite informative.
Hope this helps!