Drupal 7+, depreciated in favor of pathologic.
Path Filter takes internal Drupal paths in single or double quotes, written as e.g. "internal:node/99", and replaces them with the appropriate absolute HTTP URL or path using Drupal's url() function. It also works for files in your files directory using Drupal's file_create_url() function. It works with clean URLs enabled or disabled as well as working equally well for single site or multisite installations.
The motivation for this filter was to provide a robust way of linking to internal URLs from within content, so that your links do not break if you move your site to a different path (e.g. from a development site at http://example.com/dev/ to a production site at http://example.com/).