Qt is a C++ toolkit for application development.  It lets application
developers target all major operating systems with a single application
source code.

Qt provides a platform-independent API to all central platform functionality:
GUI, database access, networking, file handling, etc.  The Qt library
encapsulates the different APIs of different operating systems, providing
the application programmer with a single, common API for all operating systems.
The native C APIs are encapsulated in a set of well-designed, fully
object-oriented C++ classes. 

WWW: http://qt.nokia.com/products/qt/index.html