Merging several answers from above into an easy to use function
function GetScriptIfNotLoaded(scriptLocationAndName)
{
var len = $('script[src*="' + scriptLocationAndName +'"]').length;
//script already loaded!
if (len > 0)
return;
var head = document.getElementsByTagName('head')[0];
var script = document.createElement('script');
script.type = 'text/javascript';
script.src = scriptLocationAndName;
head.appendChild(script);
}