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.
A port of Qt for [http://maemo.org 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
- Qt Developer/Labs Forums http://labs.trolltech.com/forums/
- Other developer community resources http://trolltech.com/developer/community-resources including for example http://qtcentre.org