The link module provides a standard custom content field for links. With this module, links can be easily added to any content types and profiles and include advanced validating and different ways of storing internal or external links and URLs. It also supports additional link text title, site-wide tokens for titles and title attributes, target attributes, CSS class attribution, static repeating values, input conversion, and many more.
Drupal 7: Fields API is provided already by core [no dependencies].
Drupal 8: Link module is in core now. No module installation needed. Yay! Don't forget to activate it. It's deactivated by default.
INFO Since some misleading user reports we need to clarify here - Link module is NOT about to add links to any menus or the navigation nor primary/secondary menu. This can be done with default menu module (part of Drupal core). The Link module provides an additional custom field for storing and validating links to be added with any content type, which means another input block additional to your text-body, title, image and any other input you can make on new content creation.
Computed Field is a very powerful field module that lets you add a custom "computed fields" to your content types. These computed fields are populated with values that you define in PHP code. You may draw on anything available to Drupal, including other fields, the current user, database tables, you name it. (Feeling the power yet? :) ) This field is literally the Swiss Army knife of fields. So start cooking up your PHP based values!