Dolphin Smalltalk is a nice Smalltalk System for Windows. Version 7 is fully open source with MIT license on GitHub. Read more
The sources for both the image and the virtual machine will be available.
For those who do not know it: Dolphin is a Smalltalk system for Windows provided by Andy Bower and Blaire McGlashan from http://www.object-arts.com.
The existing Dolphin 6.2 codebase was brought by Andy Bower now under Git and compiles completely under VisualStudio Community 2013. This is a new version, Dolphin 7, that is modelled off the previous X6.1 Professional Edition (although you can choose to build Dolphin Community Edition from it if you like). (from http://astares.blogspot.de)