Qt (pronounced like "cute) is a cross-platform application framework, which was originally created and developed by Trolltech. Nokia acquired Trolltech in June 2008, and renamed it to Qt Software as a group within Nokia.
Qt is available for Windows, Mac OS, Linux/X11, Windows CE, Embedded Linux, and soon for S60 as well. The S60 support was announced in Symbian Smartphone Show in October 2008. An early tech-review version for early adopters is available.
There are great instructions available, if you want to get started with cross-platform development with Qt.
Although Qt is built around C++, you can use Qt APIs from other languages as well
- Developer community resources http://trolltech.com/developer/community-resources
- Qt Developer/Labs Forums http://labs.trolltech.com/forums/