Squid Web Proxy Cache
Official site. FAQs, configuration guide, support documentation and downloads. [Unix, Linux]

squidGuard
A fast redirector, and access control filter for Squid. Also provides a large collection of blacklists for blocking of objectionable sites, and a robot for generating them.

Squid: A User's Guide
Oscar Pearson's guide to installing, configuring and running a Squid implementation.

Squirm
A flexible regex based redirector for Squid.

Cuttlefish
Open source, web caching proxy server.

RU/2: Squid 2 for OS/2
Maintenance page for OS/2 version of Squid.

Squid-cache.org: Hypermail Archive
All squid mailing list entries from current day back to May 1996.

Devshed.com
Article on tuning Squid on Linux for maximum performance and stability.

Transparent proxying with squid and pf
Instructions on how to use OpenBSD packet filter (pf) to redirect http connections to a squid cache transparently without any client configuration. [OpenBSD]

Squid: The Definitive Guide
Advertising site for a book on Squid, written by Duane Wessels. Offers chapter summaries, reviews and purchase locations.

Ad Zapping With Squid
A redirector for squid that intercepts advertising (banners, popup windows, flash animations, etc.), page counters and some web bugs (as found).