With Facebook, Twitter, Google+, LinkedIn, and other websites already well established, we often help new network driven sites out by including features which allow people to sign up and sign in to the website utilizing their existing profile information they already have elsewhere.
Community websites for groups to organize and communicate with unique features
Recruiting driven websites and sites with database searching, linking, and other features
Websites requiring user profiles and database lookups
Private communities for clubs, groups, and others looking to socialize online