If one wanted to jump in and write one, what's a good language to write a compiler for, provided the resulting thing would be of some use to the developer (and maybe to others) and not a toy? Coffescript? Scheme? Or just write a parser for, e.g. Markdown?
Personally, I would make something up. If you want it to be useful, make it into an embeddable scripting language. But if you're doing this to learn, I wouldn't worry too much about making it "useful", at least not the first draft.
This is the approach I'm taking; I got into compilers because I had ideas about languages. YMMV
