[Almad]

Disclaimer: I work for Apiary

I'd say that just for getting started, hosted solution works better ;)

Anyways; we love OSS and we are gradually open-sourcing more and more from our toolbelt. Parser is completely open-source (current one https://github.com/apiaryio/blueprint-parser as well as the upcoming version https://github.com/apiaryio/snowcrash ).

More is coming soon ;)

(But yes, if you prefer writing JSON over Markdown...)