Plupload is an upload handler that allows you to upload files using HTML5 Gears, Silverlight, Flash, BrowserPlus or normal forms, providing some unique features such as upload progress, image resizing and chunked uploads. It has a powerful API that will allow you to further customize it.
Plupload is developed by the team of TinyMCE, and under GPLv2 license, so that you can use it freely in your projects.