Montessori.org
A good resource for info about the Montessori system, and Montessori herself. It also is the home of Tomorrow's Child, a monthly magazine for parents, children and teachers of the Montessori system.

The Materials Company of Boston
Top quality Montessori materials manufacturer

Nienhuis Montessori
A supplier of Montessori teaching materials worldwide.

Bruins Montessori
Suppliers of Montessori materials; ship worldwide.