Plupload - Multiple file upload utility using Flash, Silverlight, Google Gears, HTML5 or BrowserPlus!
Uppy - The next open source file uploader for web browsers
Uploadify - Uploadify is a jQuery plugin that allows users to easily add multiple file upload functionality to their website.
Uploadcare - File uploading, media processing & content delivery for modern web apps
FancyUpload - digitarald:Harald K - Consultant & Contractor - FancyUpload - Swiff meets Ajax - Swiff meets Ajax for powerful and elegant uploads.
Filestack - Simple file uploader and robust APIs for uploading, transforming, and delivering any file into your app. Filestack is a collection of tools and powerful APIs that make it simple to upload, transform, and deliver content.