How do you rename the "save" comment button?
For Drupal 7, you need to create a custom module that implements hook_form_FORM_ID_alter() using code similar to the following one (replace "mymodule" with the short name of the module you are writing):

function mymodule_form_comment_form_alter(&amp;$form, &amp;$form_state) {
  if (isset($form['actions']['submit'])) {
    $form['actions']['submit']['#value'] = t('Post');
  }
}


comment_form() uses the following code, to define the form buttons:

  // Only show the save button if comment previews are optional or if we are
  // already previewing the submission.
  $form['actions'] = array('#type' =&gt; 'actions');
  $form['actions']['submit'] = array(
    '#type' =&gt; 'submit', 
    '#value' =&gt; t('Save'), 
    '#access' =&gt; ($comment-&gt;cid &amp;&amp; user_access('administer comments')) || variable_get('comment_preview_' . $node-&gt;type, DRUPAL_OPTIONAL) != DRUPAL_REQUIRED || isset($form_state['comment_preview']), 
    '#weight' =&gt; 19,
  );
  $form['actions']['preview'] = array(
    '#type' =&gt; 'submit', 
    '#value' =&gt; t('Preview'), 
    '#access' =&gt; (variable_get('comment_preview_' . $node-&gt;type, DRUPAL_OPTIONAL) != DRUPAL_DISABLED), 
    '#weight' =&gt; 20, 
    '#submit' =&gt; array('comment_form_build_preview'),


For Drupal 6, the code should be the following one:

function mymodule_form_comment_form_alter(&amp;$form, &amp;$form_state) {
  if (isset($form['submit'])) {
    $form['submit']['#value'] = t('Post');
  }
}


I added the if (isset($form['submit'])) {} part because in Drupal 6, comment_form() defines the form buttons using the following code, and the button you are trying to change could not be present in the form.

  // Only show save button if preview is optional or if we are in preview mode.
  // We show the save button in preview mode even if there are form errors so that
  // optional form elements (e.g., captcha) can be updated in preview mode.
  if (!form_get_errors() &amp;&amp; ((variable_get('comment_preview_' . $node-&gt;type, COMMENT_PREVIEW_REQUIRED) == COMMENT_PREVIEW_OPTIONAL) || ($op == t('Preview')) || ($op == t('Save')))) {
    $form['submit'] = array(
      '#type' =&gt; 'submit',
      '#value' =&gt; t('Save'),
      '#weight' =&gt; 19,
    );
  }

  $form['preview'] = array(
    '#type' =&gt; 'button',
    '#value' =&gt; t('Preview'),
    '#weight' =&gt; 20,
  );