This is a list of programming books with open licenses, like Creative Commons, GPL, etc. Despite the posts’ titles, there are links to other resources apart from books.
UPDATE: I haven’t read the list thoroughtly, but I think there are non-free resources linked in it 😡 . So, be carefull using this material and double-check the licenses!
UPDATE2 + WARNING: one of the resources listed there is booktraining.net. ¡Don’t go to that web! Not only that site share lots of contents with non-free licenses (so it’s presence in the list is misleading) but browsing that site I almost got infected with malware!