The underlying system we use is LaTeX to generate the equations, however a lot of manipulation and configuration is needed to first get LaTeX to generate the equations in the style we want, and then to convert this into a suitable graphic for display on the internet.
LaTeX is freely available under the GNU GPL license, and we only provide instructions on how to install this.
Our proprietary code (which we make) is concerned with controlling the process automatically, checking for example that LaTeX isn't crashed or hacked, and generally converting a request from a website into latex script that can converted into a graphical image. This isn't trivial
A free alternative is LaTeXRender: http://www.mayer.dial.pipex.com/tex.htm