Drupal-ajax-tabs is a project mainly written in PHP and JAVASCRIPT, it's free.
[OBSOLETE] Drupal improved ajax_tabs module.
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The module ajaxifies MENU_LOCAL_TASK links.
Ajax Load (http://drupal.org/project/ajax_load)
Originally authored by doublethink (http://drupal.org/user/255384). The patch, fixes and improvements against 6-x.0.1 release has been done by Sepehr Lajevardi (http://drupal.org/user/668010).