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.
A port of Qt for Maemo (Hildon) is also 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