Evercookie is a plugin to track users with persistent cookies. More information on: http://en.wikipedia.org/wiki/Evercookie
evercookie produces extremely persistent cookies in a browser. Its goal is to identify a client even after they've removed standard cookies, Flash cookies (Local Shared Objects or LSOs), and others.
evercookie accomplishes this by storing the cookie data in several types of storage mechanisms that are available on the local browser. Additionally, if evercookie has found the user has removed any of the types of cookies in question, it recreates them using each mechanism available.
Specifically, when creating a new cookie, it uses the following storage mechanisms when available:
Standard HTTP Cookies
Local Shared Objects (Flash Cookies)
Silverlight Isolated Storage
Storing cookies in RGB values of auto-generated, force-cached PNGs using HTML5 Canvas tag to read pixels (cookies) back out
Storing cookies in Web History
Storing cookies in HTTP ETags
Storing cookies in Web cache
Internet Explorer userData storage
HTML5 Session Storage
HTML5 Local Storage
HTML5 Global Storage
HTML5 Database Storage via SQLite
The WordPress plugin "evercookie" is based on Samy's original Evercookie from http://samy.pl/evercookie/.