Not that any of you are likely to have heard of it, but RealBasic has one of the best cross-platform solutions available. All standard controls are set up between platforms so that source can be compiled for Windows, Mac OS, or Linux with minimal system-specific code (and in many cases, absolutely none needed) and the exact same layout and functionality. Of course controls will have the standard look of the OS they're running on, so there will be slight visual differences.
Easy to use, too. Me, I cut my teeth playing with the BASIC integrated into the command-line of a Commodore 64 computer, so I like it. It's a rapid development platform like C# or Delphi.
It's not open source or free though. *shrug*