I'd really love a web service where I can do this for various kinds of UML. All desktop apps I've used for this are just awful.
Although I find that the server will start to choke on medium to large sized diagrams, but if you are just using it to generate the image manually it works well. I currently am pointing a media wiki plugin at this server and 2 out of 10 times the server will choke on a diagram and just by retrying the request it will work.