Google Reader uses Continuations
(via BadMagicNumber)
It appears that the Google RSS Reader uses web continuations, much like frameworks like Cocoon or Seaside.
The magic: <gr:continuation>COT3ruq0jYIC</gr:continuation>
Using continuations for Ajax is a great way to store state, and you preserve the ability to “undo” an action. Continuations in a non-Ajax setting allow you to have “undo” capability built into your webapp, just by pressing the “Back” button. Traditionally, pressing the back button on a browser in webapps can be dicey at best, but if the previous state of the system as a whole was preserved, then you have nothing to worry about :)