Login to external server from Drupal site
This depends what you mean by external server? There's a tonne of contribution modules at the moment that support a variety of external logins.

You can implement Open Auth 2 to allow a single sign on:

https://www.drupal.org/project/oauth2_authentication


Or you can use Facebooks Oauth authentication: 

https://www.drupal.org/project/fboauth