This isn't strictly speaking a tutorials site, but what it does have is some cool extras for MODX sites, developed and supported by Mark Hamstra, a long time MODX specialist.
New and exciting extras being added all the time. Go check it out!
Bob Ray has contributed to MODX in many ways, including snippets, tutorials, blog posts and his book "MODX: The Offical Guide". Lots of tutorials, advice and great knowledgebase.
The Coding Pad
On Mary's blog you will find a range of tutorials and guides about many MODX Extras. There are also tuts on Evolution.
Revo In The Cloud
Susan Ottwell has been a long supporter and advocate of MODX and her site has been dedicated to articles, demos, tips and tricks to support and enhance your Revo experience.