Example of a custom module - this one extends the Domain contrib module to create a custom block with a region select.

update_domain.module

<?php

  function update_domain_enable() {
    domain_bootstrap_register();
  }

  function update_domain_domain_bootstrap_full($domain) {
    if (function_exists('drush_verify_cli') || function_exists('update_prepare_d7_bootstrap')) return;

    module_invoke('update_domain', 'initialize', 'domain');

  }

  function update_domain_init() {
    $currentkey = $GLOBALS['_domain']['domain_id'];
    $current = domain_get_domain();

    $key = $_COOKIE['labnetRegion'];
    $preferred = domain_lookup($key);

    if(isset($preferred) && isset($key)) {
      if($key != $currentkey && ($preferred['valid'] || user_access('access inactive domains'))) {
        if(!drupal_is_front_page()) {
          if ($key != 0 && $key != -1) {
            setdomaincookie($key);
            domain_goto($preferred);
            domain_set_domain($key, FALSE);
            cache_clear_all();
          } elseif ($key == 0) {
            setdomaincookie($key);
            $update = domain_default();
            domain_goto($update);
            domain_set_domain($key, FALSE);
            cache_clear_all();
          } elseif ($key == -1) {
            $key = 0;
            setdomaincookie($key);
            $update = domain_default();
            domain_goto($update);
            domain_set_domain($key, FALSE);
            cache_clear_all();
          }
        } else {
          setdomaincookie($key);
          domain_goto($preferred);
          domain_set_domain($key, FALSE);
          cache_clear_all();
        }

      } elseif ($key == $currentkey) {
        if($key == 0 || $key == -1) {
          if($key == -1) {
            $key == 0;
          }
          setdomaincookie($key);
          $update = domain_default();
          domain_goto($update);
          cache_clear_all();
        } else {
          setdomaincookie($key);
          domain_goto($current);
          domain_set_domain($currentkey, FALSE);
          cache_clear_all();
        }
      }
      if($key == 0) {
        drupal_set_message(t('Does this page seem to be missing content? Please choose your region by selecting from the options below.'));
      }
    }
  }

  function setdomaincookie($newkey) {
    $cookie = $_COOKIE['labnetRegion'];
    $params = session_get_cookie_params();
    $cookie_domain = 'labnetinternational.com';
    if(isset($cookie)) {
      if($cookie != $newkey) {
        setcookie('labnetRegion', $newkey, 0, $params['path'], $cookie_domain, $params['secure'], $params['httponly']);
      }
    } else {
      setcookie('labnetRegion', $newkey, 0, $params['path'], $cookie_domain, $params['secure'], $params['httponly']);
    }
  }

  function update_domain_select_region_form($form, &$form_state) {
    $options = array();
    $company = 'Labnet International, Inc.';

    foreach (domain_domains() as $data) {
      ($data['domain_id'] == 0) ? $key = -1 : $key = $data['domain_id'];
      if ($data['valid'] || user_access('access inactive domains')) {
        if($data['domain_id'] != 4) {
          $cleandomain = rtrim(str_replace($company, '', $data['sitename']));
          $options[$key] = $cleandomain;
        }
      }
    }

    $default = ($GLOBALS['_domain']['domain_id'] == 0) ? -1 : $GLOBALS['_domain']['domain_id'];

    $form['region_help_text'] = array(
      '#type' => 'markup',
      '#prefix' => '<div id="region-help-box">',
      '#suffix' => '</div>',
      '#markup' => '<h3>Choose Your Region</h3>',
    );


    $form['select_region'] = array(
      '#type' => 'select',
      '#options' => $options,
      '#default_value' => $default,
      '#attributes' => array(
        'onChange' => 'document.getElementById("update-domain-select-region-form").submit();'
      ),
    );

    $form['submit'] = array(
      '#name' => 'region_submit',
      '#type' => 'submit',
      '#value' => t('Go'),
      '#attributes' => array(
        'style' => 'display: none',
      ),
    );

    return $form;
  }

  function update_domain_select_region_form_submit($form, &$form_state) {
    $key = $form_state['values']['select_region'];

    if ($key == -1) {
      $key = 0;
    }

    $newdomain = domain_lookup($key);


    setdomaincookie($key);
    domain_goto($newdomain);
    domain_set_domain($key, FALSE);
    cache_clear_all();
  }

  function update_domain_block_info() {
    $blocks = array();
    $blocks['select_region'] = array(
      'info' => t('Region Select'),
    );
    return $blocks;
  }

  function update_domain_block_view($delta = '') {
    $block = array();
    $block_content = array(
      'html' => array(
        '#type' => 'markup',
        '#markup' => '',
      ),
      'form' => array(),
    );
    $markup = '';
    $regionform = drupal_get_form('update_domain_select_region_form');

    switch($delta) {
      case 'select_region':
        $markup = drupal_render($regionform);
        break;
    }

    $block_content['html']['#markup'] = $markup;
    $block['content'] = $block_content;

    return $block;
  }