Given the ubiquity of Google and its services--such as search, Google Maps, and Gmail--the company can gather an enormous amount of information about you. Google claims that the information is kept anonymous and can't personally identify you, but many privacy advocates, such as the Electronic Privacy Information Center (Epic.org), beg to differ. Firefox add-on Google Alarm (free) can help.
Google Alarm won't actually protect your privacy from Google. Rather, it shows just how frequently information about you is sent to Google servers. Whenever information about you is sent, an alarm sounds, and information is flashed on screen--what types of Google services are being used at that point, and the number and percentage of the sites you've visited that send information about you to Google.
You don't have to visit a Google-owned Web site in order for information to be sent to Google. You can visit a site that uses any of Google's services, such as Google Analytics or Google AdSense, among others. Use this software and you'll be amazed at how frequently Google gets information about you, often when you don't expect it. But the software can't actually do anything about that happening; it just alerts you that it happens.
Note that if the sound of the alarm drives you crazy--and since it sounds like an air-raid siren, it certainly will--you can download the "workplace-friendly" version from the same page. It does everything that Google Alarm does, except for making your officemates dive under their desks to "duck and cover."
If you're determined to stay away from sites that send information about you to the search giant, Google Alarm will tell you which sites to avoid. Of course, given how many sites send information back to Google, that may not be practical. At a minimum, though, it's a constant reminder of just much of your information leaves your control--and goes under Google's.