David MacKay has put the textbook he wrote online for everyone to download in a variety of formats. If you find it useful, consider buying a copy.
The full text of The Handbook of Applied Cryptography, one of the best textbooks available in the field. The original authors have gotten permission from the publisher to put the text online as sets of PostScript and .pdf files for anyone to download and share.
How to train yourself to dream lucidly.
The classic text Structure and Interpretation of Computer Programs from MIT has been put online under a Creative Commons License.