Existing APIs - No.
New API's - No, as they are only for open platform until OAuth implementations are in place to use externally.
And you are correct, you can not code your own API's.
You can ajax request page content and all pages with json=true and you can read that json data if you want.
can you please tell me how to read the JSON data?
It is JSON data.. Staple of programming.
Depending on the codebase your coding your application form it will be different. you have the cross domain policy issue with Ajax of course, but python for example would be an import as json, swift you can do it, .net..... All depends.