Pligg is an open source clone of Digg.  I have not developed for it yet, but a buddy has been hacking on it for a while.  He just put up a test version of the latest iteration of the software at OIOplus Pligg.  Yes, it has the default template in place, but what is going one behind the scenes is what is important in this deployment of Pligg.  He has implement Gzipping of the JS and CSS files in Pligg, which has made some major improvements in load times.  He has it setup for testing, so if you want to try it out, it is setup for anonymous users - you don’t have to register.  Just be aware the stories may vanish at anytime while testing.  While I have been testing I have seen a couple of cool features that I might try and integrate into the deepmarket - the nice part of open source software!