I want to develop a desktop app to be used cross-system (win, mac, linux), is there a GUI framework that would allow me to write code once for all 3 platforms and have a fully-s
What about using the Chromium Embedded Framework.
It's open-source (BSD License) and comes with bindings for C, C++, .NET(1 or 2), Java, Delphi and Python.
This .NET binding is also fully compatible with Mono which will be helpful to have a cross-platform app.
Note: AppJS, which is mentionned in another answer, is build on top of CEF (using node.js).