You didn't include bots, but I'll explain my reasons anyway. - While most people would associate a bot with nefarious activities like denial of service attacks, it's really just a generic piece of software that performs actions for or on behalf of someone of something else (pretty analogous to software agents, IMO). On the web, this could be something like an RSS aggregator, which could certainly be used on bungie.net - Zombie is often used to describe compromised systems which may take part in [distributed] denial of service attacks. Given that that is certainly possible (and might be happening right now, just on such a small, unnoticeable scale), it would be nice to be able to differentiate between legitimate requests and zombies' (though that's probably difficult). - AFAIK, there's no difference between spiders and crawlers other than what someone might want to call it. Google calls theirs the "Googlebot".