Nova is, as the title suggests, an open source client developed in PHP utilizing the GTK+ toolkit. Its a 100% interpreted application so whenever you edit the source you can instantly run the application and see the results. Nova is based on GnucDNA so it supports partial file sharing as well as all the other cool features seen in Gnucleus. Coming soon: support for the edonkey network (via mldonkey).
The Nova project page: