Sakai is the most ambitious open source project for education that I'm aware of. With the recent release of version 2.2.0, it appears that it may be ready for people to start testing. Sakai is for the high-end user. It comes with every feature that one can think of. Simpler needs are best handled with Moodle.
"The Sakai Project is a community source software development effort to design, build and deploy a new Collaboration and Learning Environment (CLE) for higher education. The Project began in January, 2004.
The Sakai Project's primary goal is to deliver the Sakai application framework and associated CMS tools and components that are designed to work together. These components are for course management, and, as an augmentation of the original CMS model, they also support research collaboration. The software is being designed to be competitive with the best CMSs available.
The tools are being built by designers, software architects and developers at different institutions, using an experimental variation of an open source development model called the community source model (see below). To provide a support system for institutions that want to be involved in the Sakai Project, either by adopting Sakai tools or by developing tools for inter-institutional portability, the Sakai Project has also formed the Sakai Educational Partners Program (SEPP) and the Sakai Commercial Affiliates Program.
The Sakai Project has its origins at the University of Michigan and Indiana University, where both universities independently began open source efforts to replicate and enhance the functionality of their existing CMSs . Soon after, MIT and Stanford joined in and, along with the Open Knowledge Initiaitive (OKI) and the uPortal consoritum, and a generous grant from the Mellon Foundation, they formed the Sakai Project. "