Let you easily integrate an OpenID login to your website. Several famous providers are preconfigured so that your users can choose whether they want login with an Google, Yahoo, etc… account.
OpenID has the advantage that your users dont have to create a new account on your website instead they use an OpenID to login. Probably more users use your website because they dont have to follow a registration process. More informations about OpenID can be found at http://openid.net.
The interface is complete customizeable so you can integrate it easy in your current design. See the Live Preview for an example.