Web sites are embedding bugs into your computer that track you even after you have cleared your browser's privacy settings.
These bugs are called Flash Cookies, or Local Shared Objects. Web sites use them to track you because they know that most people don't know about them and that they can track you even after you have cleared your privacy settings.
Web sites that use Flash cookies to bypass users' cookie privacy settings should be publicly flogged.
I went to buy some business cards on Visatprint.com and noticed very weird behavior from the site.
Even though I had logged out, cleared cookies since my last visit to Vistaprint, my name would appear on the home page of the site whenever I visited it.
Here is a screenshot of the home page, with a key to the numbers: