In addition to the MIT OpenCourseWare materials I wrote about the other day, there are many other explicitly educational web sites and web-based programs out there. A few recent finds in space technology, astronomy, and physics:
The Past and Future of Rocket Engine Propulsion - This is labeled as an "online seminar" with six sections, each of which is basically an illustrated web page. It's a pretty good semi-technical overview of the history of rocketry (Robert Goddard shown here) and of the technology of conventional and electric propulsion (electrothermal, electromagnetic, electrostatic). It is part of the Fathom Archive, a collection of free content developed by member universities.
The Search for Life: an Introduction to Astrobiology - Another good fathom.com seminar-type offering, with six brief sections.
The Mechanical Universe and Beyond - This video-based course is a bit older (circa 1985), but classical physics hasn't changed much in 22 years, and based on watching one 30 minute segment, this is quite good stuff with really clever demonstrations in the lab and in the real world. There are 52 lectures at the level of an undergrad college physics course for non-majors.
For astronomy in general, there are many on-line tutorials and even degree programs, some listed here. One of the best is Astronomy Notes by Nick Strobel, which I have written about before. And finally for space flight, JPL's Basics of Space Flight is great, especially if you do it on line and take the chapter quizzes to make sure you are paying attention!