There are three ways to accomplish this, however it is important to remember that you must retain reference to the Joomla! copyright in your source code and not remove it completely. What this means is that you must not strip every single reference to Joomla! from your code, but you ARE permitted to remove the PUBLICALLY VISIBLE TEXT in the footer area. Joomla 1.0.x:

1. Edit your index.php file to not call/include includes/footer.php

2. Edit includes/footer.php to display the information you wish to show

3. Edit the language file or the includes/version.php file that includes/footer.php calls from to show your information The problem with the last two options above is that these are core Joomla! files and hence may be overwritten by future updates.

Joomla 1.5.x:

1. Navigate to Languages>en-GB (or whatever language you're using) and find the appropriate mod_footer.ini file

2. Edit the following section:

FOOTER=Footer
FOOTER_LINE1=Copyright © %date% %sitename%. All Rights Reserved.
FOOTER_LINE2=Joomla! is Free Software released under the GNU/GPL License.
MOD_FOOTER=mod_footer
THIS MODULE SHOWS THE JOOMLA! COPYRIGHT INFORMATION=This Module shows the Joomla! Copyright information

Or alternatively some templates may have the following line in their index.php file:

<?php echo JText::_('Powered by') ?>