bookmarkable simple web editor. Simple Copy paste the code below in your browser's adress bar. Tested (pretty quickly though ^_^) on latest versions of FF, Chrome on OSX and windows

notepad.html

data:text/html;charset=utf-8,
<title>Custom Editor</title><link rel="shortcut icon" href="http://elmike.net/favicon.ico"/>
<style type="text/css">
.e{
  position:absolute;
	top:20px;
	right:50%;
	bottom:20px;
	left:0;
} 
.c{
	position:absolute;
	overflow:auto;
	top:20px;
	right:0;
	bottom:20px;
	left:50%;
	background:white;
}
.topbar{    
	background: none repeat scroll 0% 0% black;
    padding: 0px;
    margin: 0px;
    font: 13px Arial,sans-serif;
    position: absolute;
    top: 0px;
    width: 100%;
    left: 0px;
    right: 0px;
    bottom: 0px;
    height: 20px;
}
.savebtn {
    background: none repeat scroll 0% 0% black;
    padding: 0px;
    margin: 0px;
    margin: 0;
    font: 13px Arial, sans-serif;
    text-align: center;
}

.bottombar {
   background: none repeat scroll 0% 0% black;
    padding: 0px;
    margin: 0px;
    font: 13px Arial,sans-serif;
    position: absolute;
    width: 100%;
    left: 0px;
    right: 0px;
    bottom: 0px;
    height: 20px;
}
</style>
<div class="topbar">
<button class="savebtn" style="border:none; color:grey;" onClick="saveToFile()">Save as...</button>
<!--
	For other languages, you can add values like in the <select> mode element below
	eg: python, c_cpp, scala, etc... from more examples  check out https://github.com/ajaxorg/ace-builds/tree/master/src-min-noconflict. The supported languages
	are mode-mylanguage.js
	
	----------------------------------------------------------------------------------------
	For other languages, you can add values like in the <select> theme element below
	eg: eclipse, dreamweaver, github, for more example checkout  https://github.com/ajaxorg/ace/tree/master/lib/ace/theme
	
	
	-------------------------------------------------
	<!-- Press Ctrl+M or Command+M to convert your markdown code to html :) -->
	<!-- Press Ctrl+L or Command+L to hide the markdown convertion element :) -->
-->
<span style="border:none; color:grey;" class="theme">Theme:
	<select id="theme" onchange="changeTheme(this)" size="1">
	<option value="chrome">Chrome</option>
	<option value="solarized_dark">Solarized-dark</option>
	<option value="solarized_light">Solarized-light</option>
	<option selected value="twilight">Twilight</option>
	<option value="monokai">Monokai</option>
	</select>
</span>
<span style="border:none; color:grey;" class="mode">Syntax: 
	<select id="mode" onchange="changeMode(this)" size="1">
	<option value="coffee">CoffeeScript</option>
	<option value="css">CSS</option>
	<option value="curly">Curly</option>
	<option value="html">HTML</option>
	<option value="java">Java</option>
	<option value="javascript">JavaScript</option>
	<option value="json">JSON</option>
	<option selected value="markdown">Markdown</option>
	<option value="powershell">Powershell</option>
	<option value="python">Python</option>
	<option value="text">Text</option>
	<option value="textile">Textile</option>
	<option value="xml">XML</option>
	</select>
</span>
</div>
<div class="e" id="editor"></div>
<div class="c"></div>
<div style="border:none; color:grey;" class="bottombar">Inspired by <a style="color:crimson;" href="http://tsi.github.com/acelet/">Acelet</a> and  <a style="color:crimson;" href="https://gist.github.com/jdkanani/4670615">this code</a> - Powered
    by <a style="color:crimson;" href="http://ace.ajax.org" target="_blank">Ace</a> and <a  style="color:crimson;" href="https://github.com/coreyti/showdown"
    target="_blank">Showdown</a>
</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 saveToFile() {
	window.location = "data:application/octet-stream," + escape(e.getSession().getValue());
	}
    function showResult(e) {
	consoleEl.style.display='block';
	consoleEl.style.left='50%';
	editorEl.style.right='50%';
    consoleEl.innerHTML = e;
	}
	
	function changeMode(mode) {
    e.getSession().setMode("ace/mode/" + mode.options[mode.selectedIndex].value);
	}

	function changeTheme(theme) {
    e.setTheme("ace/theme/"+ theme.options[theme.selectedIndex].value);
	}

	function hideResult() {
	consoleEl.style.display='none';
	editorEl.style.right='0';
	}
    var e = ace.edit("editor");
    e.setTheme("ace/theme/monokai");
    e.getSession().setMode("ace/mode/markdown");
	var editorEl = document.getElementsByClassName("e")[0];
    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
    });
	e.commands.addCommand({
    name: "clear",
    bindKey: {
        win: "Ctrl-L",
        mac: "Command-L"
    },
    exec: function (t) {
			hideResult()
		},
    readOnly: true
	});
	
	hideResult();
</script>

This code can be copied in browser address for testing