This site has a great listing of computer-related books that are available for free on-line. I'm surprised by just how many books are listed. The books aren't just for programming, they cover computer science, Unix/Linux, databases, networking, web topics, microsoft topics, and of course programming... just about every language you can think of (yes, even COBOL).

Just choose a topic from the home page, they present of list of available books with links to the pages where the free copies are available. I tried out several of these, and all the links worked... it seemed too good to be true, so I had to check.