Light (A-to-Z Science, Discovery School): The World Book Encyclopedia article covers the basics of light.

Light and Optics (WW2010, University of Illinois): Students can learn about light and optics by studying "atmospheric optics," including phenomena like rainbows, sunsets, and halos.

Make a Splash with Color (The Tech): This online exhibit explores many aspects of color, including the visible spectrum and sight. Appropriate for younger students.

Optics (ExploreScience.com): These simple multimedia activities explore color, prisms, and lenses. Requires Shockwave™ plugin.

Science Trek: Electromagnetic Waves (Physics 2000): This interactive explanation of electromagnetic waves is part of a larger site that introduces many of the concepts of modern physics.

Spectra from Space (Exploratorium): This exhibit looks at different spectra of electromagnetic waves, putting visible light into a broader context. Includes hands-on activities, like Build Your Own Spectroscope.

Information about Lasers
Laserium: This commercial site includes an interesting section on how lasers work and a photo gallery.

Lasers (How Stuff Works): This site provides a good introduction to how lasers work.

Lasers: Science Fiction to Everyday Life (The Tech): This introduction to lasers is appropriate for younger students.

Miscellaneous
Circles of Light: The Mathematics of Rainbows (Curriculum Initiative Project, University of Minnesota): Calculus students can analyze rainbows from the mathematical side.

Einstein Revealed (NOVA Online): This unit helps students understand Einstein's major contributions to science.

Leopards: The Nocturnal Eye (NOVA Online): Students learn about light and sight by exploring an animal's nocturnal vision.

Too much light can be a bad thing. Students can learn about light pollution from this Riverdeep Current article, "Infinite Points of Light." They can also visit the Light Pollution Awareness Website.

MicroWorlds (Advanced Light Source, Berkeley Lab): This site for advanced physics and chemistry students explores what the Advanced Light Source is and how it can be used to promote materials research. Includes teacher resources.

A Spark...of Brilliance (The Franklin Institute Online): The story of how Thomas Edison turned electricity into light is one worth retelling.

The Complete History of the Discovery of Cinematography (Paul Burns, film historian): The history of cinematography parallels the history of optics.