Everyday, Packt Publishing put up one of their books as free.
Since I came about it some weeks ago via information from a fellow Microsoft Most Valuable Professional (MVP) I have included it in my daily to do list to visit the page everyday.
I have managed to get a couple very useful books. But on some days the free book is on a subject I have no interest in at all and I skip claiming a free book that day.
On top of that, they usually run promos that make available at very cheap price a book you love to have but don't want to wait till its offered free on a future date. Currently, there are running a Skill Up promo. You can get any book or video training material on the site for just $5.
The only catch is that they deal only with technical and programming books.