Easily one of the most asked questions in Joomla existence. This is probably one of the easiest ones to change as well.
While it is best to leave this message on your site as it helps spread the Joomla love, sometimes your clients do not want to partake in the love fest.
In Joomla 1.5.xx if you want to remove the footer, go to Extensions > Module Manager and unpublish the footer module. If you want to change the text, you need to get out your FTP client and do some changes.
Go to the language directory, go to the folder of the language you want to change, (in our case English), then find the mod_footer.ini. Search for “powered by” and change the text to whatever you want your text in the footer to be.
In some cases the template you are using might have it hard-coded in, or done differently. In this circumstance you should ask the developer of the template you are using.