Gliffy is a web application that fills the gap in open source office software: a Visio replacement. Sadly, it’s not open, nor is it really free. You can get a free account if you’re not using the documents commercially and even then, you can only have 5 active documents in Gliffy at any one time and they’re all public (that is, for the world to see).
Still, as a web-based application, it is surprisingly full-featured and it’s perfect for getting some diagrams quickly sketched out. Of course, if you’re really serious about your diagrams and want them to translate into code or be validated as correct UML, you will still need something like Microsoft Visio or ArgoUML (which is open and free by the way).
Gliffy supports JPG, PNG and SVG export. This is nice since it allows you to quickly save your work for use on a website, or even import the SVG into Visio or some similar product. If you know of other alternatives to Visio, preferably open or free, feel free to let me know. I’m aware of Dia, but that one just doesn’t do the trick. There’s too much focus on making diagrams that fit the rules and not enough “diagram sketchpad” for me.