Drupal: Autocomplete for any CCK text field or any form field

At present no module is available for CCK autocomplete field but you can achieve this by custom code : Edit 03-March-2012: Check out http://drupal.org/project/autocomplete_widgets

Step 1 : Use hook_form_alter(&$form, &$form_state, $form_id) and create a case for your $form_id. Step 2 : Add '#autocomplete_path' to the field where you want to add autocomplete functionality. Take reference of code given below:

<?php
$form['my_autocomplete_field'] = array(
  '#title' => t("Autocomplete Textfield"),
  '#type' => 'textfield',
  '#autocomplete_path' => 'autocomplete/example/textfield',
);
?>

Step 3 : And in hook_menu():

<?php
$items['autocomplete/example/textfield'] = array(
  'page callback' => 'autocomplete_example_textfield',
  'access callback' => TRUE,
  'weight' => 1,
  'type' => MENU_CALLBACK,
);
?>

Step 4 : Now define the callback function:

<?php
function autocomplete_example_textfield($string) {
  $items = array();
  $query = db_select('node', 'n');
  $value = $query->fields('n', array('nid', 'title'));
  $value = $query->condition(db_and()->condition('n.status', 1)->condition('title', '%' . db_like($string) . '%', 'LIKE'))
    ->range(0, 10) // Limit results for performance
    ->execute();

  foreach ($value as $val) {
    $items[$val->title] = check_plain($val->title); // Use title as key and value for autocomplete
  }
  print drupal_json_output($items);
  exit();
}
?>

Source: http://drupal.org/node/1117562