Lambda the Ultimate

You can download Epigram itself and follow the tutorial. It's worth trying.

The part that most impressed me is the interactivity. I really like this halfway point between a structure editor and a free text editor.

I think this approach would be great for designing parsers as well, where you have the content to be parsed in a separate buffer, and you interactively build the parser and see which bits gets parsed.

Also, dependently typed languages are very nifty.