Education - developing and sharing knowledge; its organization and distribution, like degrees, curriculums (curricula!), lessons; multimedia content, text books; discussions, tools..
Government - massive legacy monolith in serious need of public participation: to build new features, refactor, improve efficiency of the decision-making process, better tracking of performance issues, cost/spending, transparency and accountability. The open-source model seems particularly appropriate here.
Social infrastructure - research, development and distribution of information for alternative/affordable/sustainable ways to build machines, equipment, farming, housing, renewable energy.. Projects to help people improve their conditions in "developing" countries.
Scientific advances from outside traditional contexts like academia and corporations - more ways for amateur scientists and serious hobbyists to build, invent, experiment, collaborate.