Squid does comes with various auth modules. Take a look at the following FAQ:
Internet Access Control (IAC) is a LAMP system that integrates with Squid to deliver real-time graphical statistics, billing, monitoring and more.
A Perl script that makes cache manager requests to extract useful information, which is then stored in a number of RRD databases. A sample CGI script shows how to view the RRD data through an Apache server