How to add a popup window to website?

Hi guys, does anyone know a good JS popup window that loads when the website is opened? We want to put for special announcements. Thank you.

You can use the standard Bootstrap modal component. Simply give it a unique ID (myModal) and then use this script to call it on page load

$(document).ready(function(){
    $("#myModal").modal('show');
});

Thanks Printninja. I'll try this.