Handbook of Applied Cryptography
cconnell writes "The Handbook of Applied Cryptography is now available free (for personal use) on the Internet. This is a $100 book. Note also the companion C source code for most of the crypto algorithms, written by James Pate Williams. There is some very cool code here!"
Perhaps it's not enforceable, and perhaps it's not even legal -- but that's not the point. Why not respect the wishes of the authors who have been good enough to make the content available for free?