More than interpreter ...SGI allows to define and interpret any undeterministic 2D shape grammar. It supports real-time subshape detection and labeled rules. GUI is designed for easy manipulation with shape grammars: creation of shapes, rules and generation of designs.
Some screenshots and the current version of the Shape Grammar Interpreter can be found on the project site. Software is available for Windows, Linux and Mac OS and can be found on the download page.
Following is the presentation video for the new version of software.
The old version can be found here.