I've been using dillinger for a while, and I'm very happy with it: http://dillinger.io/
Also, it is open source: https://github.com/joemccann/dillinger
Also, it is open source: https://github.com/joemccann/dillinger
It's great to be able to run it locally too, so I can run it offline! But how do you open another file when you're offline? (So no dropbox etc)