Free Domain Availability API
You'll need your API key every time you make a request.
You can find your unique API key inside the "My account" section once you register.

You are allowed to make 1 request per minute, for more requests, support and help with integration visit WhoAPI.
Request
GET
http://api.freedomainapi.com/?domain=freedomainapi.com&r=taken&apikey=YOUR_API_KEY
Output in JSON format (domain availability example - "taken")
{ "status": "0", "taken": 1 }