JsFiddle alternative in 56 lines of Ruby. Supports Coffeescript/Sass/Haml.

    Error: Showdown is not defined
    (n)@http://2879097.run-a-gist.herokuapp.com/application.js:8
    d((function ($http) {var showdown;showdown = new Showdown.converter;return {restrict: "E", compile: function (tElement, tAttrs) {tElement.html("<article class='markdown' ng-bind-html-unsafe='markdown(" + tAttrs.model + ")'></article>");return function (scope, element, attrs) {scope[tAttrs.model] = "";$http.get(attrs.src).success(function (data) {return scope[tAttrs.model] = data;});return scope.markdown = showdown.makeHtml;};}};}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:26
    ((function ($http) {var showdown;showdown = new Showdown.converter;return {restrict: "E", compile: function (tElement, tAttrs) {tElement.html("<article class='markdown' ng-bind-html-unsafe='markdown(" + tAttrs.model + ")'></article>");return function (scope, element, attrs) {scope[tAttrs.model] = "";$http.get(attrs.src).success(function (data) {return scope[tAttrs.model] = data;});return scope.markdown = showdown.makeHtml;};}};}),0,[object Array])@http://code.angularjs.org/angular-1.0.0rc10.min.js:35
    l([object Array],(function (a) {"use strict";try {var f = b.invoke(a);if (P(f)) {f = {compile: A(f)};} else if (!f.compile && f.link) {f.compile = A(f.link);}f.priority = f.priority || 0;f.name = f.name || d;f.require = f.require || f.controller && f.name;f.restrict = f.restrict || "A";e.push(f);} catch (j) {c(j);}}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:6
    ([object Object],(function (a, c) {"use strict";b.error.apply(b, arguments);}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:35
    d((function (b, c) {"use strict";var e = [];l(a[d], function (a) {try {var f = b.invoke(a);if (P(f)) {f = {compile: A(f)};} else if (!f.compile && f.link) {f.compile = A(f.link);}f.priority = f.priority || 0;f.name = f.name || d;f.require = f.require || f.controller && f.name;f.restrict = f.restrict || "A";e.push(f);} catch (j) {c(j);}});return e;}),[object Object])@http://code.angularjs.org/angular-1.0.0rc10.min.js:27
    ([object Object])@http://code.angularjs.org/angular-1.0.0rc10.min.js:28
    c("markdownDirective")@http://code.angularjs.org/angular-1.0.0rc10.min.js:25
    z([object Array],"markdown","E",(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:43
    G([object HTMLUnknownElement],[object Array],[object Object],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:38
    w([object Proxy],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:38
    w([object Proxy],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:38
    w([object Object],(void 0),[object Object],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:38
    o([object Object])@http://code.angularjs.org/angular-1.0.0rc10.min.js:36
    ([object Object],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:15
    ((function () {"use strict";b.data("$injector", g);c(b)(a);}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:83
    ((function () {"use strict";b.data("$injector", g);c(b)(a);}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:83
    ([object Object],o,[object Object])@http://code.angularjs.org/angular-1.0.0rc10.min.js:15
    d((function (a, c, g) {"use strict";a.$apply(function () {b.data("$injector", g);c(b)(a);});}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:26
    mb([object Object],[object Array])@http://code.angularjs.org/angular-1.0.0rc10.min.js:15
    dc([object HTMLDocument],mb)@http://code.angularjs.org/angular-1.0.0rc10.min.js:15
    ()@http://code.angularjs.org/angular-1.0.0rc10.min.js:155
    a([object Event])@http://code.angularjs.org/angular-1.0.0rc10.min.js:111
    (a,0,[object Array])@http://code.angularjs.org/angular-1.0.0rc10.min.js:22
    l([object Array],(function (a) {"use strict";try {a.call(b, c);} catch (e) {}}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:6
    ([object Event])@http://code.angularjs.org/angular-1.0.0rc10.min.js:22

    http://code.angularjs.org/angular-1.0.0rc10.min.js
    Line 58

I made a website with some of the same goals in mind that takes it quite a bit further than this. That being said, I can't even come close to pretending its less than 56 lines of code.

Check out: http://plunker.no.de

Some neat features:

* Live collaboration using 'streams'

* Plunks run as first-class web citizens letting AJAX and company work out of the box

* Syntax hilighting using ajax.org's amazing ace editor

* jsHint

* Live compilation/rendering of coffee-script/markdown and I forget what else

The full source of the running app is at: https://github.com/ggoodman/stsh

Also a ground-up rewrite on http://plnkr.co has its code on: https://github.com/filearts/plunker