Here is a nice little Javascript function to load some Javascript libraries:

function includeLibs()
{
var libs = [{object: 'Prototype', src: 'http://www.mobypicture.com/slideshow/prototype.js'}, {object: 'Lightbox', src: 'http://www.mobypicture.com/slideshow/lightbox.js'}];
for (var k = 0; k < libs.length; k++)
{
try
{
var obj = eval(libs[k].object);
}
catch (err)
{
var obj = false;
}
if (obj == false)
{
var newjs=document.createElement('script');
newjs.type='text/javascript';
newjs.src=libs[k].src;
document.getElementsByTagName('head')[0].appendChild(newjs);
}
}
}
This is particularly useful if you’re writing a widget that requires external libraries but the user might already have them loaded, such as Prototype.