Bootbox

Official Website

Bootbox.js is a small javaScript library which allows you to create programmatic dialog boxes using Bootstrap modals, without having to worry about creating, managing or removing any of the required DOM elements or JS event handlers.

Sweetalert

Official Website

Here’s a comparison of a standard error message. The first one uses the built-in alert-function, while the second is using sweetAlert. SweetAlert automatically centers itself on the page and looks great no matter if you're using a desktop computer, mobile or tablet. It's even highly customizeable, as you can see below!