Mortensen Math was developed by Jerry Mortensen (V.J. Mortensen) while working as a Montessori elementary teacher in the 1970's. It is also called More Than Math.
Photos of standard Montessori lessons modified to be done with Mortensen blocks:
Mortensen Math Direct
This is the official Mortensen Math website, but the site is hard to navigate and many of the sections don't seem to be working.
Crewton Ramone's House of Math
This is the website of Ben Rogers who worked as a teacher trainer under Jerry Mortensen. There are several videos available for free, and many additional videos are available with a password ($5/1 month, $10/3 months, $15/6 months, $24/1 year).
There is also a blog with addition information and videos.
Math-U-See is based on Mortensen Math, but the lessons are more structured and formal. They also more closely follow the order of skills as they would be taught in traditional schools. It was developed by Steve Demme after being trained in the Mortensen Math method. This is widely available as a homeschool curriculum.
This is the website of Geoff White who developed his own teacher's handbook for teaching Mortensen Math ($79).
Mortensen Instead essay by Judith Townsend