It is usually a good practice to enable users to sign-in/sign-up using popular social service accounts like Facebook, Twitter, Google or OpenID.
Such an alternative not only fastens the process (no approval e-mails required, fewer form fields to fill, etc.) but also a "one less password to remember" from the user's perspective.
There are many social authentication providers besides the ones counted above including LinkedIn, Hotmail or Yahoo and integrating each can be a headache. Also, such codes may require to-be-updated from time-to-time to reflect any changes applied to them.
OneAll is a web service easing this process by providing a single API for using APIs of ~20 social services.
With the help of a "Social Login plugin", a widget that can be inserted to web pages with a copy-paste, users can be powered with an option to login with their social IDs.
A "Social Share plugin" empowers users to share their comments, purchases, reviews or other activities directly from the website with their friends on multiple social networks.
And, every action performed using OneAll (sign-in, sign-up, repeating visits, etc.) can be viewed in detail from its analytics interface.
It is hard to find such a service for free, however, OneAll is currently in beta (in-search of testers) and free during that period. Also, there will be a free plan once it is out-of-beta.
Update (18 Nov 2011) – There is now a WordPress plugin for the service which allows visitors to comment and login with social networks.