Free Science Books
webcache.googleusercontent.com
webcache.googleusercontent.com
http://physicsbooks.reddit.com
http://people.math.gatech.edu/~cain/textbooks/onlinebooks.ht...
Open Culture has many ebooks and online courses available for free.
If not, then I prefer archive.org: http://web.archive.org/web/20130424014250/http://physicsdata....
However while not free nor digital, my approach is to buy earlier editions of standard texts from Amazon. The reason I suggest this approach is that I compared the Biochemistry book in this list against my preferred text, and in my individual case the 9 or dollars for a Stryer 3rd Edition is more than worth the price differential.