Astronomy Picture of the Day: Each day this site features a different picture related to Astronomy. Some pictures are actual photographs while others are artists' conceptions. The site also contains a brief discussion of each picture's content as well as an archive of past Astronomy Pictures of the Day.
History of Physics (AIP - American Institute of Physics): An excellent and exhaustive resource of the history of Physics.
Photographs taken by the Hubble Telescope: This website contains a collection of the amazing photographs taken by the Hubble Telescope.
Periodic Table of Elements: The main page of this site displays the Periodic Table. Click on one of the elements in the table in order to go to a webpage containing information on that element. Links on a sidebar will take you to other webpages containing even more information on that element.
Powers of Ten: This webpage contains a truly amazing and exceptional instructional animation dealing with the topic of length scales.
Space Flight: This NASA website is an excellent resource for photographs and information regarding past and present US space flights.