The Squeak VM
The Squeak Virtual Machine broadly follows the specification of chapter 27 of the Blue Book, which is available at http://users.ipa.net/~dwighth/smalltalk/bluebook/bluebook_chapter27.html.
It is written in Slang, which is a functional subset of Smalltalk which is translated into standard C. Squeak essentially uses the C language as a cross-platform equivalent of assembly language.
Since Slang is a subset of Smalltalk, the Squeak virtual machine can be edited and debugged by running it in Squeak itself.
The virtual machine can also be extended with plugins, written in either C or Slang. These are used in Squeak for such things as playing MPEGs and for Squeak's built in public key encryption abilities.