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 true if you're applying writemaths to something like TinyMCE, which lives inside an iframe.