Microsoft's Office 365 APIs can be integrated into 3rd party applications. In order to do so, the app must use Azure AD authorization to confirm accounts. This page describes how to add Microsoft Azure Active Directory common consent to an application written in ASP.NET app.
Guerilla Mail is a simple idea with one purpose: to avoid spam. The idea is dead simple: go to the website, it gives you a random e-mail address that's good for one hour, and links to nothing else. Register for sites or whatever with it, and then never worry about it again. Although the web service is convenient, sometimes it's not everything you need. For those cases, one might want to look into using the Guerilla Mail API.
A Digg community member, suspicious of some top links, used the site's Digg API to uncover a 159 fake accounts. By comparing the stories voted on by these accounts to other stories, he discovered what appeared to be directed fraud and what Digg now calls "tests to find spam vulnerabilities." We spoke to the community member to learn how he used the site's API and what he learned.
Swipebuster is using the private Tinder API to obtain Tinder user data. Swipebuster can find out if a specific person is using Tinder and can provide the most recent date that person used the Tinder app. Other companies that have had private APIs reverse engineered include Uber, SnapChat, and Tesla.