If you use Adwords, they know your marketing plan and they know your purchasing patterns.
If you use Adsense, they know which of your sites makes money (though we know nothing sadly), they know how to target which ads to your site, they know how much to payout and how much to keep.
If you use Alerts, they know what topics are important to you.
If you use Analytics, they know which sites you control and/or monitor, how those sites are doing and every trend possible.
If you use Blogger, they know what you write about. Every word, every phrase, every out and in link.
If you use Calendar, they know where you have been, are, and plan to be.
If you use Catalog search/Froogle, they know what items are of interest to you and which items you actually purchase.
If you use Checkout, they know all of your personal information: name, address, phone, credit card, ccid.
If you use Desktop, they know what you have on your PC.
If you use Docs and Spreadsheets, they know that you are writing a paper on 13th century france, and your checking account is $30 overdrawn.
If you use Earth, they know where on the planet you desire to research.
If you use Finance, they know what stocks (and other instruments) you own, which you monitor, and which trends you follow.
If you use Gmail, they know everything. Yep, everything.
If you use Groups, they know you have a fetish for rare steaks and love art from Paris.
If you use Image search, they know that you like Britney Spears and you have a craving for chocolate babaka and cat photos.
If you use Local search, they know where you are now, and what you are interested in.
If you use Maps, they know where you might be, where you might be going, where you have been.
If you use Search (any Google search), Google knows every search you have ever made.
If you use Talk, they know who your buddies are.
If you use Toolbar, they know every web site you visit.
If you use Translate, they know that you are learning German.
If you use Video, the same applies as for YouTube.
If you use YouTube, they know every video you have watched, what genres you like, which naughty videos you have watched, which ones you commented/favorited on, and the videos you have uploaded.