import flash.net.URLRequest;
import flash.net.navigateToURL;
var url:String = "http://site";
var request:URLRequest = new URLRequest(url);
try {
navigateToURL(request, '_blank'); // second argument is target
} catch (e:Error) {
trace("Error occurred!");
}
//Or use this short version
navigateToURL(new URLRequest("http://site"));