How About This:

A development environment that lets you edit your language as you edit your program in that language.

The language is defined with a grammar that you can edit.  That grammar is simultaneously used to parse your program.  You can also define bells and whistles like syntax highlighting in the grammar.

Has it already been done?  Wait, I already know the answer, so I'll change the question:  Where can I download it?

fuck off with your lex/yacc/flex/bison comments because that's clearly not what I'm suggesting

>>23
Just use Lisp. You can define macros to do crazy shit; just look at the keywords you can use in a loop construct. You can even define new reader macros, so you add your own syntax by programming parsing rules for special characters. This is how people add things like { dictionary: "syntax" to: 'Lisp } using a couple dozen lines of code.

What you're asking for already exists, and it's in Lisp.