Yes, it is free. But, you ask, why would anyone give away a product that has taken hundreds of hours to build? How does anyone make money from this?
Well, the development team are first in line in getting paid employment because they have the best knowledge of the whole system. They will often be asked to set up the product they develop and will charge normal commercial rates for their services. They also have a vested interest in making the product as easy to use and set up as they can.
Lower down the chain developers who install the software get to iron out the bugs and feed back new ideas and suggestions to the development team. If anyone discovers a problem there is a huge team of developers around the world ready to help sort it out. Customers benefit from an up to date system that is being constantly improved. There are no commercial reasons to hold back development as there is with traditional commercials systems.
One of the best-known examples is the operating system that run a large proportion of the Internet itself the Apache web server and Linux operating system. It has been proven to be robust and well maintained throughout the world and is superior in many ways to its commercial rivals.