Initially developed by Design Science (now acquired by Wiris) to support rendering and displaying Math equations written in MathML for Internet Explorer, MathPlayer is a tool that plays a critical role with regards to Math accessibility on the Web. In fact, many different browsers (Mozilla Firefox, Internet Explorer, etc.) and assistive technologies leverage this tool to convey their users the semantics represented by formulas written in MathML: unfortunately, not all browsers and operating systems support this important specification natively.
More specifically, quoting its documentation:
MathPlayer can make documents more accessible by providing a means for assistive technology such as screen readers and screen magnifiers to speak, navigate, and convert to braille math in those documents. As an example, MathPlayer works with NonVisual Desktop (NVDA) to provide access to the math in Firefox, Internet Explorer, Word, and PowerPoint for Windows.
While being no longer supported, MathPlayer still remains free to use. No actual alternative to replace it is available yet, even if a very promising open source solution (called Mathcat) is being actively developed.