Online MathJaX editor I suspect this might easily be a clone question, it seems impossible that nobobdy would've asked this before, but my searches seem to return nothing.
I've grown very used to the MathJaX input here. So much, that when I need to show some maths to somebody, I'd go to Math.SE "Ask Question" and then just type it up there and then usually send it as a picture or something along those lines.
This is, however, very impractical (not to say lame)
My question therefore is: is there any way how to do this in a more sophisticated and practical way? Either output the Math.SE preview in a better way, or a (not necessarily) online editor. I've tried many different online editors, but they all fail at different details such as recognizing stars as bold/italic, characters such as $\mathbb{R}$ or $\mathscr{U}$, general formatting, etc.
Thanks!

Edit: I wouldn't want to turn this into a LaTeX online editor question. I am well aware of that and I can always give up and go to LaTeX. It's just that I love the input on Math.SE and would like to use it not just for asking questions here.
To demonstrate further - I sometimes take an existing answer on Math.SE and make edits on it (and/or translate it) for my own purposes. What would be the easiest way to do this?
 A: Most sites offer no assistance in entering formulas, these ones do:
Excellent online mathjax editor with clean interface
http://www.texpaste.com/
Other options, interface a bit more complicated though but that's a personal choice


*

*http://www.codecogs.com/latex/eqneditor.php

*arachnoid.com/latex/ 

A: For later reference - this question is now obsolete, as https://stackedit.io/ exists.
edit: as mentioned in the comments, StackEdit has switched from MathJax to KaTeX
A: The meta site has a sandbox for drafts. You can type your post there and share a direct link to it.
A: [Last update 2015-05] Just to add some options for people stumbling upon this.

*

*stackedit comes closes to regular StackExchange-like Markdown and MathJax behavior.

*marx works similarly.

*MathBin is focused on sharable snippets. The already mentioned MathB.in has been retired and points to texpaste which does have markdown support.

*Notepag.es's website appears dead, but you can find the source on GitHub if you want to host it yourself.

*Authorea can also do markdown with MathJax.

*Qute is a desktop application. There are now a gazillion others and many text editors have plugins (e.g., Atom, Sublime)

All of these should work for markdown+mathjax but might not behave identical to the SE-flavored combination.
A: I use MathBin to send quizzes or notes to students.
