What are some free proxies out there that could do blacklisting (& possibly greylisting)
as well as auto-block by know malicious sources (eg: get updates from SpamHaus,
AlienVault, bad Reputation sites & known sources of malwares).
Ideally the free proxies could also stop users from downloading executables or
a specified file types. No plan to go for commercial ones like Bluecoat.