Note: content extracted from Creating your own Authentication Provider
One of the extension types that was added in the last couple of years was the Authentication Provider. Unlike most other providers, an authentication provider is not configured in the web.config and actually looks and behaves more like a module than an actual provider.
An authentication provider installs up to 3 UI elements that the framework cares about – a login control, a settings control and optionally a logout control. It is certainly possible to use more controls if needed, but these are the only three the core framework interacts with directly. Once installed the authentication provider can appear as one option in the standard DNN login
There are a number of alternative implementations provided within the core and via 3rd parties, these are listed below:
The 6.2.0 release of DotNetNuke added twitter, live, facebook and google providers.
3rd party providers
A number of these are free, open source, codeplex hosted modules, others are available on the author's sites or via the DNN Store.
Building your own provider
For more detail on creating your own provider please see Creating your own Authentication Provider