Celtix is free because it is "open-source". That means, somebody out there probably wrote a small neat little program, and wanted the world to notice, benefit from it, and contribute to it, so he/she made the actual code available to everyone on the planet.
An open-source licence means that using the software is free, but if you change the code to suit your needs, you are obliged to release the new version and the new code to the world so that everyone benefits like you did. In this manner, the tools simply gets better and better over time. The secret is, no single person updates the software, everybody and anybody can. Its a collaborative effort.
Ever heard of Linux. Its an entire Operating System that is OpenSource. www.Sourceforge.net has hundreds of open-source software for your downloading or contributing.
