http://www.freetechbooks.com/
As you'd expect, the list includes a lot of stale and/or not-very-valuable titles, but there's also a lot of genuinely worthwhile stuff there. Judging from their "most popular" list, the site is a big hit with C++ programmers. But there are also 23 free Java books, and lots of timeless reference material for programmers of all stripes.
