Many browsers support MathML and support in math&science applications is ubiquitous. To write MathML, you don't need more than a text editor, but there are many tools available that make it even easier.
Of course, nearly all software has bugs and some programs are further ahead implementing MathML 3 than others.
- Microsoft Windows Math Input Panel (handwriting recognition)
- Microsoft Equation Editor
- OpenOffice.org Math editor
- WIRIS editor
- ItexToMML (math mode only)
- GNU TeXmacs
- NATbraille (MathML to Braille)
- pMML2SVG (MathML to SVG)
- lasem (MathML to SVG)
- pmml2tex (MathML to TeX)