Write maths, see maths is a jQuery plugin for editable areas which gives an instant MathJax preview of the TeX you're writing just above the cursor.
To use it, just call
$(selector).writemaths()$ when the page loads.
You can optionally pass in an object with some options:
cleanMaths: a function which does something to the LaTeX before displaying it.
callback: a function which is called when the preview is displayed.
iFrame: set this to
trueif you're applying writemaths to something like TinyMCE, which lives inside an iframe.
The old version of write maths, see maths was an attempt at a line-by-line WYSIWYG editor with instant maths preview and a load of other stuff. It lives on at takenot.es.