Free online textbooks.

As most of these are written on a college level, they are, of course, advanced materials. However, if you have a high schooler interested in a particular topic/subject, or perhaps you are yourself, you might want to check out some of these offerings.

FREE TECH BOOKS - covers computer science, programming & scripting, mathematics, and other related fields

FREELOAD PRESS - mostly maths and computers, but a few other books are available, such as "Writing Exploratory Essays". Just note - the site says "most" of the books are free, but some have a end-users fee.

WIKIBOOKS - covers a very wide range of topics, languages, sciences, history, and more. Has a section called wikijunior specifically for younger children. This site, being a wiki, is definitely a work in progress, but it does have some nice reference resources.