Pharo

Pharo is a cross-platform implementation of the classic Smalltalk system that provides the basis for Glamorous Toolkit. Pharo extends Smalltalk-80 in several ways, including support for dynamic arrays, pragmas, traits and slots.

Although GT is built on top of Pharo, it offers a completely new Graphical stack and its own implementation of

To learn Pharo, take a look at A tour through Pharo.