http://webgraphviz.com + add an on-key-up event to the text area that clicks the button = my favorite graphviz editing experience.

if you're doing anything more complicated by hand than you can achieve in that kind of setup, you're likely better off with a more sophisticated dedicated tool / manually doing all of your layout. but I have yet to see anything else get even close to beating this setup for "zero to a usable diagram" time or effort.