This article shows how to configure IdentityServer3, when you need to authenticate and authorize usage of your WebAPI/MVC, for users stored in SQL Server.
The attached open source C# .NET library is an SMTP client implementing implicit SSL and OAuth2 protocols. The library answers the question: how to send email message using Gmail as a server. Or, how to send email message using email server that implements implicit SSL and port 465.
This top 5 list is to help equip you with the tools and knowledge needed to analyze how a vendor implements OAuth2.0, how to interface with RESTful APIs, and ultimately gain access to end user data.