Top 20 NuGet auth Packages

NativoPlusStudio.AuthToken.Fis is part of the NativoPlusStudio.AuthToken set of libraries that can be used to retrieve the auth token to be able to interface with the FIS SOAP APIs.
Hawk Http Authentication for ASP.NET Web API
Google Sign-In is a secure authentication system that reduces the burden of login for your users, by enabling them to sign in with their Google account, the same account they already use with Gmail, Play, Google+, and other Google services.
A Xamarin wrapper around Xaamrin.Auth APIs to share in Social Media(Facebook & Twitter for now)
Core models for Endpointer.Accounts.
A .NET API client for Firebase Rest Authentication API that follows Google's API spec as precisely as possible.
Free and Open Source fork of ServiceStack V3 Adds OpenId Auth Providers for use in NServiceKit Web and REST Services. Includes built-in support for Google, Yahoo, MyOpenId and Custom OpenId providers. More Info on Auth Providers:
Trimble Connect Platform Services authentication flow for desktop and mobile applications.
Another Implementation of the Twilio's Authy 2FA Api for .Net Framework
CaveSystems Auth Bibliothek.
Enhanced identity management built on ASP.NET Core Identity.
This component will calculate SHA-hash for your message and then sign that with a certificate stored in Microsoft Azure KeyVault.
TwitterOAuth an OAuth library that provides the basic functions required for authorization.
A cross-platform API for authenticating users and storing their accounts.
Lightweight, Fast JWT(JSON Web Token) implementation for .NET Core.
Gitter# Auth provide you a way to authenticate user on Gitter API
Deprecated. Use the strong named ServiceStack.Authentication.OAuth2 package instead.
Provides various authenticators for user authentication in Crosslight apps, including HttpBasic, NTLM, OAuth, OAuth2, and WebAPI authenticator.
Elasticsearch data store adaptor for ASP.NET Core Identity.