Open System Architect (OSA) is an application used for modelling a system. OSA currently supports data modelling (physical and logical) with UML in the works
Being always excited to try out anything webbased and free, I was delighted to see this very useful and cool UML diagramming tool. You can even create a 'napkin style' model like it's written by hand
via <a href="http://www.dev102.com/2008/09/29/a-web-based-sequence-diagram-editor/">Dev102.com</a>