OCW - MIT. Programming eBooks. Since this post got quite popular I decided to incorporate some of the excellent suggestions posted in the comments, so this list now has more than 50 books in it.
BTW: I’m not very strict on the definition of “ebook”, some of them are really just HTML versions of books. [UPDATED: 2012-01-18] Learning a new programming language always is fun and there are many great books legally available for free online. Einztein.