Really glad you find it useful, and you're completely right that I need a section on compatibility. The nice thing is that for browsers that don't support Blobs, Amazon allows you to just upload the file as one giant chunk with the same multipart API. I'm not absolutely sure yet (I still need to do some testing), but this should mean that browsers versions as far back as IE 5 will work - only basic XHR compatibility will be required to function. I'm planning on covering as many browsers as possible, I just haven't found a great way to go about this yet. Is there a better option than http://spoon.net/browsers/?