Enter your .dot code in the textarea, leave the focus to recalculate the graph and watch the result on the right page.
I was annoyed because writing graphviz graphs was painful, but thanks to vis.js and emscripten everything can be done in the browser. This is the first idea of a quick prototyping page for graphs. Just copy the examples from the graphviz Gallery into the text editor and enjoy :). It works good with the simpler examples but has problems with radial layout. Your Browser might hang for a little because everything is rendered on the client.
This page was made using viz.js and knockoutjs and Behave.js.