Kivy
Kivy is an open source software library for rapid development of NUI apps.
Kivy is an open source Python library for rapid development of applications that make use of innovative user interfaces, such as multi-touch apps.
It can use natively most inputs protocols and devices like WM_Touch, WM_Pen, Mac OS X Trackpad, and Magic Mouse, Mtdev, Linux Kernel HID, TUIO. A multi-touch mouse simulator is included.
Kivy is 100% free to use, under LGPL 3 licence. The toolkit is professionally developed, backed and used. You can use it in a product and sell your product. The framework is stable and has a documented API, plus a programming guide to help for in the first step.
The graphics engine is built over OpenGL ES 2, using modern and fast way of doing graphics. The toolkit is coming with more than 20 widgets designed to be extensible. Many parts are written in C using Cython, tested with regression tests.