Kivy is an open source Python library for the rapid development of cross-platform user interfaces. Kivy applications can be developed for Linux, Windows, OS X, Android and iOS using the same codebase.
Graphics are rendered via OpenGL ES 2 rather than through native widgets, leading to a fairly uniform appearance across operating systems.
Developing interfaces in Kivy optionally involves the use of kvlang, a small language that supports python-like expressions and python interop. The use of kvlang can drastically simplify the development of user interface as compared to using Python exclusively.
Kivy is free to use (currently under the MIT license), and professionally backed.