The only official JS wrapper I'm aware of is http://www.salesforce.com/us/developer/docs/ajax/
However it's possible to create a SOAP string and use XMLHttpRequest or $.ajax() to make the callout. Works fine.
Looking ahead, I would end up in rewriting many functions (or all). I'm now just wondering if I'm the first one who needs it or if there is already something existent? If there is nothing, I would also like to collaborate in a new project - just in case if others would be interested in such a JS wrapper, too.
The same need finally also applies to Tooling API.