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

Ruby Notepad Bookmarklet

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

does not work on mobile safari, typing doesn’t show up

ace.js has moved. Check the CDN Library Page to get the cdn link for the latest Ace version whenever you are using this.

Here's the CDN link to ace 1.4.12: https://cdnjs.cloudflare.com/ajax/libs/ace/1.4.12/ace.js.
Code snippet with the updated link:

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>