This bookmarklet gives you a code editor in your browser with a single click.

notepad.html

data:text/html, <style type="text/css">.e{position:absolute;top:0;right:0;bottom:0;left:0;}</style><div class="e" id="editor"></div><script src="http://d1n0x3qji82z53.cloudfront.net/src-min-noconflict/ace.js" type="text/javascript" charset="utf-8"></script><script>var e=ace.edit("editor");e.setTheme("ace/theme/monokai");e.getSession().setMode("ace/mode/ruby");</script>
<!-- 
For other language: Instead of  `ace/mode/ruby`, Use

Markdown -> `ace/mode/markdown`
Python -> `ace/mode/python`
C/C++ -> `ace/mode/c_cpp`
Javscript -> `ace/mode/javascript`
Java -> `ace/mode/java`
Scala- -> `ace/mode/scala`
CoffeeScript -> `ace/mode/coffee`
and 
css, html, php, latex, 
tex, sh, sql, lua, clojure, dart, typescript, go, groovy, json, jsp, less, lisp, 
lucene, perl, powershell, scss, textile, xml, yaml, xquery, liquid, diff and many more...




For other theme: Instead of  `ace/theme/monokai`, Use

Eclipse -> ace/theme/eclipse
GitHub -> ace/theme/github
TextMate -> ace/theme/textmate
and 
ambiance, dawn, chaos, chrome, dreamweaver, xcode, vibrant_ink, solarized_dark, solarized_light, tomorrow, tomorrow_night, tomorrow_night_blue, 
twilight, tomorrow_night_eighties, pastel_on_dark and many more..
-->


<!-- If you want a markdown converter use this : !-->
data:text/html,<style type="text/css">.e{position:absolute;top:0;right:50%;bottom:0;left:0;} .c{position:absolute;overflow:auto;top:0;right:0;bottom:0;left:50%;}</style><div class="e" id="editor"></div><div class="c"></div><script src="http://d1n0x3qji82z53.cloudfront.net/src-min-noconflict/ace.js" type="text/javascript" charset="utf-8"></script><script src="http://cdnjs.cloudflare.com/ajax/libs/showdown/0.3.1/showdown.min.js"></script><script> function showResult(e){consoleEl.innerHTML=e}var e=ace.edit("editor");e.setTheme("ace/theme/monokai");e.getSession().setMode("ace/mode/markdown");var consoleEl=document.getElementsByClassName("c")[0];var converter=new Showdown.converter;e.commands.addCommand({name:"markdown",bindKey:{win:"Ctrl-M",mac:"Command-M"},exec:function(t){var n=e.getSession().getMode().$id;if(n=="ace/mode/markdown"){showResult(converter.makeHtml(t.getValue()))}},readOnly:true})</script>
<!-- Press Ctrl+M or Command+M to convert your markdown code to html :) -->

As someone else mentioned in a previous comment, ace.js has moved. Refer to this CDN directory to get the latest Ace version whenever you are using this.

Right now, the latest version is Ace v1.4.12 (the directory listing also mentions that there are no known vulnerabilities as of now but use this at your own risk) and the link is: https://cdnjs.cloudflare.com/ajax/libs/ace/1.4.12/ace.js.

Replace the src attribute in the script tag with the link you copied in the previous step. The code snippets would now become:

Simple editor

data:text/html, <style type="text/css">.e{position:absolute;top:0;right:0;bottom:0;left:0;}</style><div class="e" id="editor"></div><script src="https://cdnjs.cloudflare.com/ajax/libs/ace/1.4.12/ace.js" type="text/javascript" charset="utf-8"></script><script>var e=ace.edit("editor");e.setTheme("ace/theme/monokai");e.getSession().setMode("ace/mode/ruby");</script>

Markdown Editor

This uses showdown.js for converting markdown to html. This is the CDN directory link for showdown. Here's the current CDN link for Showdown v1.9.1: https://cdnjs.cloudflare.com/ajax/libs/showdown/1.9.1/showdown.min.js.

I have replaced both ace and showdown CDN links in the src attributes for this editable snippet.
As mentioned in the original gist, Ctrl+M or Cmd+M converts the markdown to html on the side panel.

data:text/html,<style type="text/css">.e{position:absolute;top:0;right:50%;bottom:0;left:0;} .c{position:absolute;overflow:auto;top:0;right:0;bottom:0;left:50%;}</style><div class="e" id="editor"></div><div class="c"></div><script src="https://cdnjs.cloudflare.com/ajax/libs/ace/1.4.12/ace.js" type="text/javascript" charset="utf-8"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/showdown/1.9.1/showdown.min.js"></script><script> function showResult(e){consoleEl.innerHTML=e}var e=ace.edit("editor");e.setTheme("ace/theme/monokai");e.getSession().setMode("ace/mode/markdown");var consoleEl=document.getElementsByClassName("c")[0];var converter=new Showdown.converter;e.commands.addCommand({name:"markdown",bindKey:{win:"Ctrl-M",mac:"Command-M"},exec:function(t){var n=e.getSession().getMode().$id;if(n=="ace/mode/markdown"){showResult(converter.makeHtml(t.getValue()))}},readOnly:true})</script>

is there a way to run the code