[https://addons.mozilla.org/en-US/firefox/addon/collusion/ Latest released version of the add-on]
Latest testing build is at http://people.mozilla.com/~jdicarlo/collusion.xpi (edit: where does this URL come from?)
Discussion happens on the dev-privacy@lists.mozilla.org mailing list.
The original Collusion blog post by Atul Varma: http://www.toolness.com/wp/2011/07/collusion/