Trigger an event with Prototype

event.simulate.js fits your needs.

I've used this several times and it works like a charm. It allows you to manually trigger native events, such as click or hover like so:

$('foo').simulate('click');

The great thing about this is that all attached event handlers will still be executed, just as if you would have clicked the element yourself.

For custom events you can use the standard prototype method Event.fire().

I don't think there is one built in to Prototype, but you can use this (not tested but should at least get you in the right direction):

Element.prototype.triggerEvent = function(eventName)
{
    if (document.createEvent)
    {
        var evt = document.createEvent('HTMLEvents');
        evt.initEvent(eventName, true, true);

        return this.dispatchEvent(evt);
    }

    if (this.fireEvent)
        return this.fireEvent('on' + eventName);
}

$('foo').triggerEvent('mouseover');