Why not build a web app? It will run practically on any platform that has a browser which is basically everywhere. That's basically what I do lately. Anytime I think of neat idea I first prototype it in firefox with the help of firebug and more often than not it turns out to be good enough.
I've definitely considered that as a possibility, however part of the application is to discover (search for) files on your system, and then upload specific files based on requests from other people. I was thinking that this would be slightly prohibitive in the web browser, at least from what I know from building Java applets and Flash based uploaders. If I were to do this in the browser, I would want to use purely JavaScript, but I'm unsure if this would be powerful enough for what I want to do.
