Science Mission Directorate
This directorate is responsible for all of NASA's programs relating to astronomy, the solar system, and the sun and its interaction with Earth. There are four general space science themes of NASA SMD research: Search for Origins, Structure and Evolution of the Universe, Solar System Exploration, and Sun-Earth Connections.
http://science.hq.nasa.gov

Space Science Missions
The best source for space science mission information is through the NASA Science Missions Page. Here you will find an updated and comprehensive list of past, current, and future space science missions. Missions are organized by their stage in development (under study, in development, operating, past missions) and by their mission name or acronym.
http://science.hq.nasa.gov/missions/phase.html

Space Science Education and Public Outreach (E/PO) Programs
NASA SMD Education and Public Outreach (E/PO) programs focus on K-12 education and the public understanding of science, and are embedded in every SMD flight mission and research program.
http://science.hq.nasa.gov/education/index.html

Space Science Education and Public Outreach Support Network
The NASA Science Mission Directorate has established a Space Science EPO "Support Network" to make education at all levels and the enhanced public understanding of science integral parts of the Space Science missions and research programs.
http://science.hq.nasa.gov/research/ecosystem.htm

Space Science Education and Public Outreach Support Network Brochure
NASA has put together an online brochure describing how the Support Network works. (.pdf / 133kb).
http://spacescience.nasa.gov/education/
resources/ecosystem/brochure_low_res.pdf

Aerospace Technology
The Aerospace Technology Enterprise pioneers the identification, development, verification, transfer, application and commercialization of high-payoff aeronautics and space transportation technologies. The Enterprise is managed by the Office of Aerospace Technology.
http://www.aero-space.nasa.gov/

Biological and Physical Research
The mission of this Enterprise is to conduct basic and applied research to support human exploration of space and to take advantage of the space environment as a laboratory for scientific, technological, and commercial research. The Office of Biological and Physical Research creates unique cross-disciplinary research programs, bringing the basic sciences of physics, biology, and chemistry together with a wide range of engineering disciplines.
http://spaceresearch.nasa.gov/

Earth Science
The Earth Science Enterprise uses space to provide information about Earth's environment that is obtainable in no other way. In concert with research and industry partners, the Enterprise is developing the understanding needed to support the complex environmental policy and economic investment decisions that lie ahead. The Earth Science Enterprise is managed by the Office of Earth Science.
http://www.earth.nasa.gov/

Education
The Education Enterprise seeks to inspire the next generation of explorers as only NASA can. The Office of Education is a clearing house for all NASA education programs. They lists all of the NASA education resources available for educators, students, scientists, informal educators and the public.
http://science.hq.nasa.gov

Human Exploration and Development of Space
The Human Exploration and Development of Space Enterprise is to explore and to expand the human experience into the far reaches of space. The enterprise is managed by the Office of Space Flight whose programs provide safe, assured transportation to and from space for people and payloads, and develop and operate habitable space facilities to enhance scientic knowledge, support technology development, and enable commercial activity.
http://www.hq.nasa.gov/osf/