Eco

Holier Toledo! Eco is a text editor that parses arbitrary languages on the fly and allows you to insert "language boxes" to embed other languages within them. The data structure actually saved is the composite parse tree, not the text, which is mildly alarming given the reliance of the existing programming infrastructure on text source code (although Laurence Tratt, the editor's promulgator, notes that Smalltalk has solved many of these problems).

This is really quite attractive. I can't think of a good reason it couldn't be duplicated in something along the lines of Padre, using Marpa.