Yes, actually there are quite a few ways to authenticate to an Apache server/site.

On this url: http://freeradius.org/mod_auth_radius/ you will find a good explanation of how to set up an Apache website to use something called Radius authentication. There is also Basic (which was already mentioned) along with a few other methods which are typically implemented the same way.

What will happen when you set up this type of authentication is when the user goes to the website, a dialog box will pop up asking for user id and password. Once that is entered the user will have access to that site from then until they close their browser...