When the software is free, it is easier to support the market. Struts is a typical example. Although it is just a new product, there have been many articles and tutorials involve it, but there is nothing like this book.
Many development teams don't like to use software that is not internally developed. Open source components provide all the advantages of all self-developed software, but never lock you in a proprietary solution that only you know.
Open source software is a win-win situation for everyone.
