Any page that Joomla renders by default contains a meta-tag for "generator". This is to allow search engines and visitors to see what Content Management System (CMS) you are using. Although I am a "credit where credit is due" type of guy, I am also a security minded individual. I want any potential attackers and hackers to have as much trouble as possible, so I don't want them to know what CMS system I am using.
Here is what the Meta Tag Creates:
<meta name="generator" content="Joomla! 1.5 - Open Source Content Management" />
Here is how to remove the code completely from your site.
Open /libraries/joomla/document/html/renderer/head.php
Find the line -
$strHtml .= $tab.'<meta name="generator" content="'.$document->getGenerator().'" />'.$lnEnd;
Delete this line
Refresh your page, check the source and the "generator" meta tag will be gone.

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