Uploadify allows you to easily integrate multiple (or single) file uploads in your website. Basic implementation is really easy even for novice users, but it also allows advance users to completely customize it through an array of options.
It requires Flash and any backend development language of your choice. Some important customizable options include ability to specify allowed file types, file size and number of simultaneous uploads.
Developed by Ronnie Garcia & Travis Nickels; Uploadify is available for download under MIT & GPL License. You can find further information, demo & download on Uploadify’s Website.