Okay, as you can see there are only two here. I have not been able to find any others that have a good amount of traffic. Please contact me (faye@lurking-grue.org) if you know of any forums or lists I can include here.
A Montessori discussion forum with no registration required. A good forum, but low traffic.
Montessori Education Discussion List (ML) is a mailing list. Visit that URL for instructions on how to subscribe. The instructions are in a PDF document.