Dojo toolkit, is well known in the Ajax community to provide numerous widgets to web developper.
Since version 0.4.1, it provides a way to store data on the client side trough dojo.storage.
It can use multiple ways to store those data, especially flash (which is present on 97% of the computers ).
Check out the Moxie demo application that combines the dojo rich text editor and the dojo.storage method.