A simple application written in Ruby, using the Sinatra framework. Hopefully it should be easy to read even to those who don't know Ruby.
To run it, do:
rackup -I.
And the application should be accessible at http://localhost:9292.
The important bit is the client code, found at /public/index.html
This code is thoroughly commented, and there's additional information at our original article covering Ajax file uploads
Although initially I (Pablo Brasero) referred to this technique as HTML5 uploads, this is not actually correct. The interfaces used are described in two separate specifications, different from HTML5. These are XMLHttpRequest level 2 and the File API.
Also, remember that these specifications are, at the time of writing, working drafts. They could change in the future, before they become full-fledged W3C recommended standards.