Chad Whitacre wrote:
http://www.creativecommons.org/
I'm not sure I would call them an "advocate" for the public domain,
although they provide for dedications to it. I see them advocating for
everything in between (pd) and (c).
There is also the "Center for the Public Domain":
http://www.centerpd.org/
I'm sure you can find more on Google.
Ok, then I'm either searching wrong or there's nothing to find. :-)
There is lots to find. For starters, you can try these searches:
Lessig "Public Domain"
Lawrence Rosen "Public Domain"
Ulrik Petersen
I am not a lawyer.