Hi Brian,
The W3C's MathML homepage is of course a good place to start. They have a list of software using MathML. There are two portions to the MathML specification: the presentation set, which will tell a rendering engine how to typeset the equation (for instance, distinguishing between a stacked vs. beveled fraction), and the content set, which will tell calculation engines how to interpret the equation (is y(x) a function y of x or two variables being multiplied?). Most applications will focus on one subset of MathML or the other depending on the scope of the application.
MathML is being used more and more widely and with good reason - MathML has all of the advantages of XML (for more information, see our white paper MathML is XML).
Design Science makes WebEQ, which includes components for editing and displaying MathML on your website. We also have an SDK version if you would like to include these components in a desktop application. WebEQ is Java based.
Hope this helps, but if you have more questions, feel free to email [email protected].
Best wishes,
Autumn Cuellar, XML Specialist at Design Science