Sites pulling sneaky Flash cookie-snoop
“What's far less well known is that Adobe Flash software also features cookies that can be used in much the same way as HTTP cookies. Flash cookies can be used for storing the volume level of a Flash video but the technology can also be used as "secondary, redundant unique identifiers that enable advertisers to circumvent user preferences and self-help", the academics warn.
A significant percentage of websites including federal government sites use this Flash-based technology to track users, the researchers discovered. The technology is sometimes used as a means to "undelete" the information in browser-based cookies that a user might have thought they had cleared from their system when they deleted their browsing history, the academics explain.”
http://www.theregister.co.uk/2009/08/19/flash_cookies/
More info and a script here Online Privacy Tips
Added - Jim Mack informed me of the CCleaner (apparently the first C is short for cr*p) that cleans those up. I cleaned my system using the above mentioned script an hour or two ago and since found three flash cookies from a website ytimg.com for which no web page exists but is apparently for YouTube. Which makes sense as I had indeed visited YouTube in there.