US Department of Energy
Fus Ed Web: Physics of a Fundamental Energy Source
An excellent resource on nuclear fusion! Using incredible graphics and excellent discussion, this site (with its several pages) provides an exhaustive online course on the topic. Click on the link for "a Guided Tour" and begin.
Stanford University
Stanford Solar Center: About the Sun
The Stanford Solar Center provides information about the sun, activities, solar art, and solar folklore.
Harvard University
Harvard University: The Sun
Includes a brief history of the sun's development and its future. Also provides simple statistics, solar activities, and references.
NASA
Nasa: The Sun's Energy Source
This site, which is provided by NASA, gives some scientific history and a clear and detailed explanation of nulcear forces and nuclear fusion.
Other
University of Leicester: Sun
Provides a brief overview of the Sun, including a focus on solar eclipses, solar wind, and solar sailing.