Showdown is a Javascript Markdown to HTML bidirectional converter, based on the original works by John Gruber. Showdown can be used client side (in the browser) or server side (with nodejs). See it working in the live demo!
var converter = new showdown.Converter();
var md = '[**Showdown**](http://www.showdownjs.com) is *great*\n' +
'because:\n\n' +
' - it\'s easy to use\n' +
' - it\'s extensible\n' +
' - works in the server and in the browser';
var html = converter.makeHtml(md);
No hard dependencies or special installation instructions! Just download it (or use a CDN) and include it in your file.
Out of the box, supports a number of popular markdown flavors like "vanilla", GFM or commonmark. Extra features like tables, metadata, etc.. can be enabled through options.
Showdown works both in server (nodejs) and client-side (browser).
It's easily customizable and extendable, meaning you can add new syntax or modify it's behavior through extensions or options.
Showdown is compatible with both new and old (such as IE8+ and nodejs 0.12).
Written in EcmaScript5 means you can use it in your project as is, without the need of transpilers.