An authenticator that you can use in Squid is pretty simple. So if you find you need to use an authentication scheme that is not yet supported you can pretty easily write one yourself. All you need is some basic knowledge of scripting languages (bash, Perl, Python, whatever).
This article is meant to help set up LDAP authentication with a Squid proxy. Often administrators are using Squid as the central proxy for a company. For security reasons users need to enter their username and password before they are allowed to surf the internet. Firewalls usually deny accesses to the internet that would bypass the proxys. This way - if a user it doing dangerous things - you know who it was.
I once needed a way to make sure our Squid proxy cluster operates correctly. So I wrote a rather simple Squid check plugin for the Nagios monitoring software. André Moura has pimped it further. Feel free to use it. It's also available from monitoringexchange.org.
Many administrators do not seem to recognize the true purpose of a proxy. That is to make sure nothing but a certain protocol is used through the proxy. Squid is a HTTP proxy. It makes sure that people use nothing but HTTP through it.
Don't panic - this page isn't intended for spammers who look for new ways to send their crap. It's merely here to tell Squid administrators how a wrong ACL/http_access configuration can quickly turn your HTTP proxy into an SMTP (mail) spamming machine.