I am developing some personal software and use a number of operation systems, such as Linux, Windows XP & Vista and Windows Mobile. Yeah, the first name is hit on my head, Java. The virtual machine of Java can run various platform too and they has very very similar Look and Feel across various platforms. But my major language is C# at this moment. My solution is wvWidget. This a C++ GUI API library and can run in Win32, MacOS, Linux and WinCE(Windows Mobile). Moreover, it has a .Net Library. So I can develop the application via this library.