An ExpressionEngine plug-in to allow you to manually switch sites through the front end.

pi.change_site.php

<?php
$plugin_info = array(
  'pi_name' => 'Change Site URL',
  'pi_version' =>'1.0',
  'pi_author' =>'Nine Four Ltd',
  'pi_author_url' => 'http://www.ninefour.co.uk/labs/',
  'pi_description' => 'Changes the current site URL.',
  'pi_usage' => Change_site::usage()
  );

class Change_site
{
	function Change_site()
  	{
		global $DB, $PREFS, $TMPL;
		
		
		
		// Get site id
		$site_id = $TMPL->fetch_param('site_id');
		$query = $DB->query("SELECT site_name, site_system_preferences, site_pages FROM exp_sites WHERE site_id = ".$site_id);
		
		$site_system_preferences = unserialize($query->row['site_system_preferences']);

		$PREFS->core_ini['site_id'] = $site_id;
		$PREFS->core_ini['site_url'] = $site_system_preferences['site_url'];
		$PREFS->core_ini['site_pages'] =& unserialize($query->row['site_pages']);

	}
	// usage instructions
	function usage() 
	{
  		ob_start();
?>
-------------------
HOW TO USE
-------------------
	<?php
		$buffer = ob_get_contents();
		ob_end_clean();
		return $buffer;
	}
}
?>