Googlebot
Googlebot is Google's website crawling software, which spiders through websites on the internet to index websites, which then appear in Google Search.
Googlebot finds new websites by following links on other websites, but does not follow links with the rel nofollow tag.
Googlebot identified itself to the webserver with the user agent header, like so:
Googlebot/2.1 (+http://www.google.com/bot.html)
Last updated on 26 January 2009, at 14:12.