Mariana Garcia's blog has wonderful PBL resources. This post has projects for life science, earth science, physical science, engineering, and STEM competitions.
There are way too many project-based learning blogs out there to mention nowadays. It has become such a staple of modern learning. So what's a teacher to do? Great resources are everywhere on the Web. The GDCF's own Solution Fluency Activity Planner currently hosts over 20,000 users.