I remember seeing this a long time ago, but never got around to looking at them. However, when I was in the bookstore using one of the Borders 30% off coupons I came across the Bruce Perens' Open-Source Series published by Prentice Hall.
All of the books in the series are released free of charge and open-source several months after their initial publication. I'm particularly interested in their PHP 5 Power Programming (coming up from PHP 4), and Rapid Application Development with Mozilla. I'm looking forward to the Subversion Version Control which was published in May and should be out any time now.
Check out the download links below each title. They also carry titles for programming in Qt, Samba configuration, Linux development and more.