The most useful api is login/authentication which is used in most of the login site . shephertz have the solution for it. you can use user api for your login purpose you can also set user profile using api.
Why to use shephertz api?
1). Easy to use, just do registration and get your keys and start using service
2). Free use upto 1 million calls.
3). Mostly helpful for the individual developer .
4) samples are also provide to help developer how to use.