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

>>1 here, I'm probably just being trolled, but the comments about preprocessors and functions make me think that the original post was misleading.

I'm just imagining working in an editor, writing some code, and thinking "it would be nice if I had a keyword called foo that did so and so."  Then I'd just go up to the menu and click something like "switch to grammar edit view" and I could define foo and then click "switch to code edit view" and I'd be right back to editing my code, but I'd have this new keyword foo available and the code editor would highlight it as a keyword, the compiler would know what to do with it, etc...

Just adding a new keyword is a pretty trivial example.  If you had full control over the grammar, you could change everything about the way your language works.