GraphvizFiddle (viz.js 2.1.2 release, with circo/dot/fdp/neato/osage/twopi engines)

GraphvizFiddle draws DOT language [directed] graphs, see DOT User's Guide (on http://graphviz.org/).
It is an online code editor and playground for Graphviz dot layout tool, see blog posting for details.

After drawing the layout, engine and format will snap back to defaults (dot/svg).
Adding eg. "layout=neato" setting to (di)graph (for "Share" links) overrides engine selection.
Choose the engine and format you want and press "Draw".    (NEATO User's Guide, engine manpage)
Engine specific settings (eg. "overlap") don't hurt for engines not supporting them (eg. "dot").

DOT input & control   new/ʍǝu blog posting 2
initial ortho scalable number rho-221 (from Graphviz Gallery: cluster crazy datastruct fsm hello switch)