Access to Quandl Databases

You can create an account at Quandl for free or you can sign in with your GitHub, Google or LinkedIn account. According to the API of Quandl there are Usage Rules

The Quandl API is free to use. However, you must have permission to access the data resources you request. All open databases have unrestricted permissions. To access any premium database, you must be a subscriber to that database.

and Rate Limits

Anonymous requests are subject to a limit of 50 calls per day. Note that anonymous requests for premium databases are not accepted. Authenticated users have a limit of 2,000 calls per 10 minutes, and a limit of 50,000 calls per day. Premium data subscribers have a limit of 5,000 calls per 10 minutes, and a limit of 720,000 calls per day.