Google

Check sentences for duplication online via Google. For plagiarism checks with Google, Proofread Bot requires a free Google Search API Key that you can obtain a Google Custom Search (CSE) key at https://code.google.com/apis/console under "Services" option. Once you enable the Search service, you will find your key under the "Access" tab.Click here for more info on Google CSE

Google gives you 100 queries for free per day, but with a rate limit of 1 sec /. query that is quite slow! You can change your key later at http://proofreadbot.com/user.