proxycheck is a simple tool that will work on a reasonable *nix system and may be used to quickly check whenever a given host or set of hosts has open proxy server running. Open proxies of various kinds are (ab)used nowadays for various evil things like sending mass spam, hacking into your machine, making denial of service attacks (DoS) and the like. Every such machine should be either secured properly or turned off permanently, but that's not an option, since in most cases there is either no administrator of such machines exists at all, or he has no clue about what's on that machine, or it's irrelevant for him.