challengebasedlearning.org is a social networking site for educators and students interested in Challenge Based Learning (CBL) which is Apple's take on PBL. This is a page of resources created by educators.
For a quick reference of what CBL is, check out this article: https://net.educause.edu/ir/library/pdf/ELI7080.pdf
I used this resource frequently when teaching PBL focused Technology to K-5th. It has many great resources including innovative digital tools, project ideas, etc. and search functions, by content area, by project type, etc. I often referred to it when needing to come up with a new project or tool to execute a project.