Search Engines use a script called 'BOTS' or 'robots' that crawls the net for content or for some specific purpose.

Here's a list of Google bots for your reference.

Adsense: "Mediapartners-google/2.1"
Adwords: "googleBot/2.1"
google-: "Mozilla/5.0 (compatible; googlebot/2.1; http://www.google.com/bot.html)"
Image--: "googlebot-Image/1.0"

Sometimes I use this to do a count of how many adsense bots came today, or how many google search bots visited.

In unix, I have a logfile called access_log and I do queries such as

#cat access_log | grep 21/Jul | grep googlebot

This will list all instances where googlebot spider visited my site on 21 Jul. Or if I only need the count, I could just add "wc -l" so it will look like

#cat access_log | grep 21/Jul | grep googlebot | wc -l

Another useful one is to look at all visits that came from google search

# cat access_log | grep 21/Jul | grep google | grep search

With that, I can see what search terms people use to reach my site. Of course you can access some of these data easily from awstats or webstat.