I like free online tools as I use Mac, Ubunutu, Windows, iPhone and iPad at different times from different locations for same or different purposes, both office work or chess or other interests.
I use http://www.onlinehtmleditor.net/ as Online HTML Editor.
I use http://www.jsoneditoronline.org/ as online JSON Editor.
Now I wanted to come out of Visio or MS Paint or Dia for diagramming as those are offline or installed applications.
Today I tried to use Gliffy after searching and testing several other tools online. I see that it gives me close to the functionality of MS Visio although its free and online based.