https://github.com/wordnik/swagger-core/wiki/Downloads
(Node inspector recently added for command-line happiness.)
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...)
https://github.com/wordnik/swagger-core/wiki/Downloads
(Node inspector recently added for command-line happiness.)