Doh! Auth2 is a simple Sinatra app useful for debugging OAUTH2 API's. The form can be pre-populated via a defaults.yaml.
ruby app.rb
:client_id: OAUTH2_CLIENT_ID
:client_secret: OAUTH2_CLIENT_SECRET
:site: OAUTH2_SITE_URL
:auth_path: OAUTH2_AUTH_PATH
:token_path: OAUTH2_TOKEN_PATH
:scope: OAUTH2_SCOPE
:resource: API_RESOURCE_PATH