SEO Canonicalisation is a system plugin for Joomla! 1.5 that enables site 'canonicalisation', which means that when a user hits your site from another domain than your preferred (for example, www.sammoffatt.com.au instead of sammoffatt.com.au), Joomla! will redirect the user to the appropriate hostname. Previouly this would have to be accomplished by utilising a .htaccess redirect.

For more information check out the Joomla! Extensions Directory or the Joomla! Forum topic on canonicalisation.