Storing data at the client side becomes more and more important and I could not find such a module in http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/resources/jquery/, yet.
Use cases on Commons: Collecting nominations for Featured content, saving inputs, ...
If I should do something, just tell me and I try to write a patch. But I read, you will move to Git, soon so I'd better wait and learn something new ;-)
There is one dependency to /resources/jquery/jquery.json.js required.
jStorage is licensed under MIT-styled license: