As the natural extension of Gareth Rushgrove’s bookmarklets for inserting the Dojo or YUI Javascript toolkits mentioned by Simon, here’s a tidying-up of a bookmarklet I’ve been using to bring in any Javascript using user input via a popup prompt:
javascript:void(function(){
var%20s=document.createElement("script");
s.src=prompt("Full%20URL:");
document.getElementsByTagName("head")[0].appendChild(s);
}())
I was originally appending it to the body as an invisible div, making use of d.innerHTML to add both src and type attributes so as to make sure the browser won’t ignore it. The above based on Gareth’s seems to work fine, though.