This is an almost working "else" binding for Knockout.js. It monkey patches the "if" binding and keeps a stack of the last returned values from the bindings. Every "else" binding is actually an if binding with the negated value of the last if bindings return set in a preprocessor. Currently doesn't work with multiple bindings. DOH!

ko-else.js

(function(){
    var bindingKey = 'else',
        stack = [],
        
        _if = ko.bindingHandlers.if,
        _init = _if.init
        _update = _if.update;

    ko.bindingHandlers[bindingKey] = 
    {
        init: _init,
        update: _update,
        preprocess: function(val)
        {
            if(!stack.length) throw new Error("Stray 'else' binding");
            // Negate this sonsa bitch
            var last = stack.pop();
            var v = !last ? 'true' : 'false';
            return v;
        }
    };
    
    // Hijack the if binding
    ko.bindingHandlers.if.init = function()
    {
        var dataValue = ko.utils.unwrapObservable(arguments[1]()),
            result = _init.apply(this, arguments);

        stack.push(dataValue);
        return result;
    };
    
    // Virtualize this shit
    ko.expressionRewriting.bindingRewriteValidators[bindingKey] = false;
    ko.virtualElements.allowedBindings[bindingKey] = true;


})();