simplest-authentication

Using .htaccess and .htpasswd will give a quick authentication for a given folder.

You should use the pwd command to figure out the path to the .htpasswd file.

You can generate htpasswd files using

htpasswd -c .htpasswd username

for the first time and

htpasswd .htpasswd username

for any future users.

If you don't have htpasswd (like on cloud9) then you can use a tool like this htpasswd generator

This method is a simple way to protect static sites. It doesn't allow users to register (although you could do it) and (as far as I know) it doesn't allow AJAX requests through