Squeak is a full-featured implementation of the Smalltalk programming language and environment based on (and largely compatible with) the original Smalltalk-80 system. Squeak has very powerful 2- and 3-D graphics, sound, video, MIDI, animation and other multimedia capabilities -- and one of the most impressive development environments ever created. It also includes a customisable framework for creating dynamic HTTP servers and interactively extensible Web sites. The entire Squeak system is open source software, distributed freely with a liberal license.
This is the virtual machine Squeak runs on.