This snippet lists all related terms of a taxonomy term in term view.

if (arg(0) == 'taxonomy' && arg(1) == 'term' && is_numeric(arg(2)) && is_null(arg(3))) {
  $tid = (int)arg(2);
  $result = db_query(db_rewrite_sql('SELECT tid, name FROM {term_relation} INNER JOIN {term_data} ON (tid1 = tid OR tid2 = tid) WHERE (tid1 = %d OR tid2 = %d) AND tid != %d ORDER BY weight, name', '{term_relation}', 'tid'), $tid, $tid, $tid);
  $output = '';
  while ($term = db_fetch_object($result))
  {
    $output .= ((!empty($output)) ? ', ' : '') . l($term->name, "taxonomy/term/$term->tid");
  }
  return $output;
}

This block was designed to be viewed in the content region. Change ((!empty($output)) ? ', ' : '') to ((!empty($output)) ? '<br />' : '') if you want to use it in a sidebar.