Let's say that you want to select a random link within a certain div tag which has a given id and then jump to where that link points to. Here's a way to do that.

function random_link(id)
{
    var obj = document.getElementById(id);
    var links = obj.getElementsByTagName("a");
    var offset = Math.floor(Math.random() * links.length);
    document.location = links[offset].href;
}