Instigate GUI Toolkit is an Abstraction for multiple GUI Toolkits allowing to write single GUI code which can run on multiple platforms and with multiple GUI back-ends. Now it supports QT and gtkmm back-ends on Linux. Work on GTK+ and EWL is ongoing.
Sakura is a Knowledge Navigator and User Interface for UNIX, which implements HyperMedia and its own windowing and packing system, both in the main program and in an extensive API for Tcl and other languages.
ACDK - Artefaktur Component Development Kit - is a platform independent C++-framework similar to Java or C#/.NET for generating distributed and scriptable components and applications.