Sisyphus.js

I'm really impressed with sisyphus.js and its usage of HTML5 localStorage. It makes it so users won't need to re-enter data into a form when they close the window.

It's super easy to implement, you only need to include the following in your header:

<script type="text/javascript" src="sisyphus.min.js"></script>

And, you will need to activate it on the form you want to save with something like this:

$('form').sisyphus();

Source:  http://simsalabim.github.com/sisyphus/