About Rainbows - Good information on rainbows.  Most appropriate for teacher reference.

Encarta Encyclopedia - The entry for rainbow.

Rainbow Lab - Another teacher reference site on rainbows.

How Stuff Works - Answers the question, "What causes a rainbow?"

The Rainbow Maker - A site promoting an individual who creates large scale rainbows.

Rainbows - A photographers site which features beautiful rainbow photographs.

RainbowLand - A story of rainbow colors in which the ending is left out for the students to complete. 

Hooray for the Rainbow! - Lesson ideas involving a rainbow's colors.

All the Colors of the Rainbow (Rookie Read-About Science) by Allan Fowler.  Basic information on rainbows and their formation.

Eugie's Rainbow by Debbie Powell Smith.  A caterpillar finds himself on an adventure in a rainbow.

Planting a Rainbow by Lois Ehlert.  The story of planting a garden of rainbow colored flowers.

The Rainbow Bridge by Audrey Wood.  An embellished Native American legend about people crossing a "rainbow bridge" to a new mainland.

Rainbow Crow: A Lenape Tale by Nancy Van Laan.  A Native American Legend on how the crow lost his colors.

The Rainbow Fish by Marcus Pfister.  The well known story of a fish sharing his brilliant rainbow colored scales.

A Rainbow of My Own by Don Freeman.  The classic story of a boy pretending he has his own rainbow with which to play.

The Rainbow and You by E.C. Krupp.  Full of good information on rainbows, ranging from folklore to the science of how they form.

Raindrops and Rainbows by Rose Wyler.  A book of simple science experiments revolving around rain, storms, and rainbows.

A Rainy Day by Sandra Markle.  A story of rain including good information on the formation of rainbows.