Qt (pronounced "cute") is a cross-platform application and UI framework. It is the recommended development framework for app developers targeting Symbian devices and the MeeGo based Nokia N9. Qt is also available for Windows, Mac OS, Linux/X11, Windows CE, Embedded Linux.
Qt provides a number of technologies for application and UI developement. Nokia recommend Qt Quick (a declarative language) for UI development on mobile platforms along with Qt Mobility for accessing use cases that are more important in mobile devices.
There are many excellent resources for Qt developers both on this site and elsewhere: