What's the minimum required Squid config to make a public proxy server?

Although a Really Bad Idea™, just add acl all src all to the top of your ACL list.

Again, this is not a good idea whatsoever.

Like the other posters, i'm going to call this a bad idea until proven otherwise (although i can think of a couple of good reasons to do it).

Instead of the acl suggested by Nathan C i'd suggest:

• http_access allow all (instead of http_access deny all, which is the default)
• acl Safe_ports port 1-65535 (instead of the Safe_ports entries in the default config)
• acl SSL_ports port 1-65535 (instead of the SSL_ports entries in the default config)
• If you're trying to create an anonymised service, set forwarded_for off and via off.

Again, do you really want to do this?