Ever heard of Flash cookies? They're often used to track your activity. And sometimes even if you delete them they reappear, complete with session state data preserved.
According to a new whitepaper by researchers at the University of California, "We find that more than 50% of the sites in our sample are using Flash cookies to store information about the user. Some are using it to “respawn” or re-instantiate HTTP cookies deleted by the user. Flash cookies often share the same values as HTTP cookies, and are even used on government websites to assign unique values to users. Privacy policies rarely disclose the presence of Flash cookies, and user controls for effectuating privacy preferences are lacking."
To arrive at their results the researchers analyzed "HTTP and Flash cookies from the top 100 domains ranked by QuantCast results of July 1, 2009" and gathered their results on July 27.
You can (and should!) read the full report by downloading the PDF file.