There's just one problem: the API is still really lame. Yes, it's a big jump up from cookies, but that's not saying much. It's verbose, only works with strings and is decidedly lacking in features and convenience. This is exactly the kind of wheel that i cannot resist reinventing.
I actually wrote store.js well over two years ago, in its first incarnation. It has come a long way since then and has been well used here at ESHA Research. However, it has never yet seen the "light of day" out there in open source. Now, you can fork it on GitHub!
store.js handles JSON transparently, provides a very rich API, and is even extensible (with a few nice extensions and a few crazy ones already available in the repo). It supports both concise and explicit function calls to suit your needs (e.g. store('foo') === store.get('foo')). It even supports namespacing your data with ease.