If you click on the ## Currently Active Users at the bottom of the main page, it brings up a list of what everyone is viewing. There are always 5 or more Google "Spiders" lurking in the neighborhood.
So, you web experts, what do they do (I assume they are giving information to Google on topics, etc.), how come so many? and it would seem that Google must have millions of these things sneaking around the web if 5 are located in just this one site.
I also see a few spiders from other search engines at times.