Obviously if you use are a user of Google services you are sharing a lot of your search behavior among other things with Google as well. The E-Justice blog has published an interesting post on 25 Surprising Things That Google Knows About You. Below is a sample from the complete list of 25.
The web pages you visit: Google AdSense is used by many web pages for online advertising, and Google’s cookies record your visits to web pages with their ad program on them.
The blogs you read: If you use Google Reader, Google knows the blogs you subscribe to. Even if you’re not on Google Reader, Google knows all of the Blogger pages you visit.
Who and what you’re emailing: GMail users, and those who send mail to GMail users share a variety of personal and business information with Google.
What’s on your PC: If you’re using Google Desktop, Google knows everything that you keep on your computer.
Your schedule: Google Calendar opens your personal and business schedule up to the prying eyes of Google.
Your social network and interests: Google indexes sites like Orkut, Facebook, and Digg, and as such, has access to information about what you’re interested in online.