When you're creating a social site you want to make it as easy as possible for new users to register. Or even better: have them log in with one of their existing accounts so they don't have to go through yet another registration process and have yet another account to maintain. OpenID goes a long way in doing this, but people that are less tech savvy might not be familiar with it and you don't want them to read through a page of documentation just to sign up. Just an example: most users don't realize they can enter the url of their Google profile and use it as an OpenID url.

However, most users that participate in social networking sites already own a Twitter, Facebook, MySpace or Blogger account. Services like Gigya (which is completely free) and RPX (which includes a free plan) provide an easy way for users to register on your site by logging in with one of their existing accounts. Gigya created a Gigya Socialize module for easy integration with Drupal. As a site admin all you need to do is sign up on Gigya (using one of your own accounts) and create an API key for your site. Enter this API key in the module configuration page and you're ready to go.

As a sidenote: apart from user authentication Gigya also plugs in to the different social APIs so that users can send status updates and tweets from your site for instance, or so they can connect with their friends from other social communities. It's like an instant social starter kit.