Gushiku Studios has posted three Box2D tutorials to Youtube. They primarily deal with applying collision filtering for interesting effects for games and apps.
The first video is a tutorial detailing how collision filtering works via category and mask bits in Box2D. You can find that video here.
The next video demonstrates an approach to implementing one-sided platforms using sensor fixtures. You can find that video here.
Finally, the last video demonstrates how to implement a railway switch type construct, similar to what you might create for a pinball game. You can find that video here.