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.
Standardization, or lack thereof, around identity, authentication and authorization for open web APIs is one of the greatest challenges to mashup application developers today. So it's quite notable that Google not only just quietly added OAuth support to their Google Contacts API but also stated that "This is our first step towards OAuth enabling all Google Data APIs."