NASA
Nasa: Innovative Engines
NASA's Glenn Research Center reports on its innovative engines program, namely ion propulsion, a technology that once only powered imaginary spacecraft in science fiction novels. It is now a reality.
Other
Spacetech's Orrery: Comets Visitors From Afar
This site provides a report on comets, with many links to more detailed information. Content includes a focus on Halley's Comet, Hale Bopp, and Shoemaker-Levy 9.
University of Wisconsin
The Why Files: Keeping an Eye on Ions
Discusses the development of a new rocket engine that operates by the ion propulsion as opposed to gas propulsion. Well written and great graphics!