Nobodu user id

Everyone has to be someone and everything has to be owned by somebody. The 'nobody' uid (and the 'nogroup' gid) are usually granted almost no privileges. You can safely use them for things like bootp servers. The reasoning is that If you allow outside users to access services which run with no privileges, they can't do any harm.

Windows has a similar account called 'IUSR_machinename' which is used for anonymous access to http and ftp servers.