ko-persist.js

ko.extenders.persist = function (target, option) {
    target.subscribe(function (newValue) {
        window.localStorage.setItem(option, newValue);
    });
    return target;
};

//and use it like this:

var item = ko.observable().extend({persist:"storedItem"});

//or if you want to also initialize the observable with the stored value:

var item = ko.observable(window.localStorage.getItem("storedItem")).extend({persist:"storedItem"});