Qt

Qt is a cross-platform application and UI framework for developers using C++. Qt can be used under open source (LGPL 2,1, version 3) terms.

In relation to Valve

Some tools available for no charge from Valve utilize Qt.

The source code for Valve's usage of Qt can be downloaded as a ZIP archive for Win32 (1.7GB) and for Win64 (1.7GB).

This drop is based off the Qt code base version 4.8.3.