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 as a group within Nokia.
Qt is available for Windows, Mac OS, Linux/X11, Windows CE, Embedded Linux, and now for Symbian and Maemo as well.
The S60 support was announced in Symbian Smartphone Show in October 2008 and now Qt 4.6 is here, offering full Symbian support. In addition, a technology preview of new Qt APIs — from the Qt Mobility project — are available, along with a second technology preview for the Maemo platform.
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