Online MathJaX editor
The best way:
For later reference - this question is now obsolete, as https://stackedit.io/ exists.
[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.
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/