Custom JS Code lets you add custom JS to the site from the administrator interface, without modifying the template's files or requiring any access to the server's FTP. Click here to get more information.
Click here to access the extension in the Joomla Admin interface.
Here are some examples of what you can do:
Example 1: put an alert on a link
Alert on a link
Example 2: load some images using a JSONP call to Flickr
Click to load images
..or any JS technique you may need (you still have to build them by pasting the code).