The Buck Institute for Education (BIE) has an excellent site about Project Based Learning (PBL). In project based learning, students work on a complex problem or question and work collaboratively to design a solution. The BIE site has sample projects categorized by subject and grade. For example, they have 27 high school world history projects that you can read and download. Here is an example of one 9th grade project called A.D.1095 and All That.