Phoca Site Plugin is a Joomla! CMS plugin. With help of this plugin, different parts of html code can be added to the site additionally. E.g. attributes (xmlns) of HTML tag can be changed, Google Analytics code can be added to HEAD tag, etc.
I came across a cool widget that needed some code in the Head of the HTML of each page. I couldn't be bothered to "hack" my template to add the code manually and then I found this nice plugin that made it very easy.