I just came across a piece of software called KFC that removes Flash Cookies AKA Local Shared Objects from your computer. These are new to me but a little googling shows a pretty consistent theme, e.g.:
(webdesign10.com)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.
(wikipedia)Several services even use LSOs as surreptitious data storage to reinstate traditional cookies that a user deleted, a policy called “re-spawning”