Science Books Online is exactly what it's name says. It is a listing of different, free, online science books.
The list is sorted by topic: astronomy, biology, chemistry, physics, math, medicine, etc. and there is a recently added section on the main page so you can see what's new.
There are free e-books, textbooks, lecture notes and other science documents. The texts are either available online or for downloading. I have used quite a few of the physics ones in my classes. They are very well written and designed.
This is a great resource for schools to either supplement their current textbooks, or maybe even replace the traditional, expensive, paper textbook with a free electronic one.