This link does require a purchase, although some sample chapters are unlocked on ASCD. It is incredibly valuable as a resource for K-12 educators, though, as several of the chapters discuss units from grades 3 to 12 in various disciplines. If you're looking for inspiration and more than just theory, it may be worth the 10 dollars to you.
Edutopia tracked Sammamish High School in Washington after they got a grant to convert their entire school from traditional delivery to problem-based learning. Not only is this a case study worth reading if you want to try to make the conversion, but the videos, coverage, and articles come directly from the teachers who are making and living through the change in their school.
Problem-Based Learning and Project-Based Learning (and similar methods) often get confused with other nontraditional classroom learning methods. This article compares and contrasts them, which can eliminate a lot of confusion and help you find the model that is right for you and your classroom.
If you need to "sell" someone on problem-based learning, this may be your best resource. Even aside from all the great lesson ideas and processes, this site goes into the research and benefits behind PBL. I love how it breaks the research down into relevant learning theory, links to research papers, and proven examples.
This video describes how problem-based learning works at a university, but it provides a good model of what a PBL class would look like at any level. It would take a lot more facilitation in a K-12 setting, but the blend of lectures for information and research sessions, with a teacher or tutor to facilitate and guide, is a good model to make the PBL more student-centered.
This is an actual sample problem-based unit plan for a science class. This unit walks teachers through what a project-based unit would look like step by step, plus it gives reasons for why things should happen to align it with problem-based learning goals. It is a good reference point to use if you are designing your own unit.
Investigative Case Based Learning is a subset of problem-based learning. Instead of a hypothetical problem devised by the teacher or a real problem, which may have limited data or practical solutions, the students use case studies as a jumping-off point for inquiries. This learning method may be more useful in science and social science courses, where case studies are common (this site uses science cases), but it is adaptable to any discipline.
Although this page refers to a workshop for business faculty, don't let it fool you into thinking it isn't useful. This page is a treasure trove of information on what problem-based learning is, its rationales, sample units, research, and links to helpful sites such as the PBL Clearinghouse, which aggregates PBL lessons for other teachers to use in various disciplines. You may need to download some of the documents to see them, but the information is incredibly useful to PBL educators.