Disclaimer: I'm working for Scolab on one of the editor listed below.
This list of MathML editors could help you:
http://www.w3.org/Math/Software/mathml_software_cat_editors.html
I know these web-based editors and display engines:
- FMath and MathMlEditor I met the guy maintaining this, he's great.
- MathJax The author told me he wanted to build an editor in JavaScript. That would be awesome, but I don't know how far he is with it. He's capable.
Alfred's Equation Editor (AlfredEq) - This equation editor was developed by us for BuzzMath and is available for licencing.
- Video of an integration of AlfredEq in BuzzMath's edition tools
Screenshot of the editor embedded in ShareMath.com:
Please post your other findings!