Hello,
I've written a jQuery plugin which, when applied to an editable area in a page, such as a <textarea> tag or an element with the `contenteditable` attribute, gives a little floating preview box above the cursor when you enter LaTeX. I've made a demo page, which has a link to the code, at
http://christianp.github.com/writemaths/. It also works on TinyMCE editors.
I've also rolled it in to a wordpress plugin which gives the preview when typing a comment on a post. I've had MathJax enabled on my blog for a while now but very few commenters have used it - I think they were dissuaded by the fact they wouldn't know if they'd got their LaTeX right until they submitted their comments. This plugin should help with that.
Thanks!
Christian