jQuery HTML5 Uploader is a lightweight jQuery plugin that lets you to quickly add an upload system a-la-Gmail into your web app. The plugin comes with drag and drop support and can handle multiple file uploads at once.
You only need to create a dropbox element (i.e. a div) and jQuery HTML5 Uploader will do the rest. Then you can drag & drop one or more files on the element and the files will be uploaded.