QT1 is a cross-platform C++ application framework released by Trolltech. Open Source versions for the X11 platform are available for those creating Open Source software.
The KDE desktop uses QT as its base technology. This table shows CTL scripts supported by QT version 3.3 on the X11 platform at the time of this writing (Jan. '05). Script support on Windows or Apple OSX may be slightly different.
Support for the complex scripts shown above (and of course other non-complex scripts present in Unicode) is completely transparent to the programmer. The programmer can, for the most part, just use QT's QString, QLineEdit, QTextEdit, QLabel, and derived classes. As QT provides a complete object-oriented application framework that is also cross-platform, it certainly represents a very attractive option for software developers and businesses looking to expand into the Linux market.
In contrast, IBM's ICU is platform-independent but cannot directly display text without having a concrete rendering class written on top of the platform-agnostic base class.
1. Trolltech's QT, http://www.trolltech.com.