I can integrate the external API's by doing the authentication and authorization by the API own documentation. Here is the list of API's that I already has integrated as follow.
1- Google Maps API
2- YouTube API
3- Google Analytics
4- Mandrill Email API
5- Twitter API
6- Instagram API
7- WooCommerce API
8- HubSpot CRM API
9- Copper CRM API
10- Pipedrive CRM API
11- Slack API