https://stripe.com/docs https://parse.com/docs http://blog.parse.com/2012/01/11/designing-great-api-docs/ http://sendgrid.com/docs http://www.twilio.com/docs https://go.developer.ebay.com/developers/ebay/documentation-tools http://www.etsy.com/developers/ https://developers.facebook.com https://www.dropbox.com/developers https://developers.inkfilepicker.com
some oauth-specific stuff that demonstrates generating an authorization and using the token to make queries:
https://developers.facebook.com/tools/explorer/
https://developers.google.com/oauthplayground/
https://developers.google.com/admin-sdk/directory/v1/reference/groups/get (click the "oauth2" slider)