For future readers: Mentioned diagramming programs/apps

http://www.diagram.ly (based on mxGraph, a javascript lib)

http://www.lucidchart.com/

http://www.gliffy.com/ (requires flash)

https://www.circuitlab.com/

http://www.umlet.com/ (download)

http://www.flowchart.com

http://docs.google.com (drawings not diagrams)

gojs.net (still in private alpha)

I'd add that I've tried:

http://www.lovelycharts.com

http://www.cacoo.com (flash)