Try Lovely Charts 😉 I tried it very very quickly and impatiently. I think it is a nice option to have and to know that it is available out there.
There are quite some web-based visual diagramming tools available, some of which have been featured at infosthetics from where I got the tip.
Lovely Charts tries to stand out the crowd of applications available by making the user-generated flowcharts, sitemaps and wireframes more beautiful than those from its competitors.