LocalStorage is one of the features of HTML5. It provides a key, value store in your browser. Here are some resources for more information.
Found a couple of free tutorials.
HTML5 Offline Application Cache
PaperKilledRock: HTML5 LocalStorage
Of course, with new features like this, new libraries are always welcome. I have not tried this yet, but I intend to.
Finally, we have a book by one of the folks working on the spec.