PD is short for Project Dogwaffle, a digital painting and animation program for PC.

A free version of PD (v1.2) which dates back about 5 years can be found at www.thebest3d.com/dogwaffle/free
There are several commercial versions, some of which have in fact been available on Renderosity a while back.

For this and many of the several commercial versions, there are also some free add-ons, plugins and standalone extensions. Here are a few:

www.thebest3d.com/dogwaffle/3rdpartyplugins
www.thebest3d.com/dogwaffle/freebies/plugins

There's also Lua-based scripting to make your own filters or share filters with your Artweaver and Pixarra twistedbrush or Gimp, through gluas.
www.thebest3d.com/dogwaffle/lua
This doggybag also adds a few such scripts.

Free tutorials are found at www.thebest3d.com/dogwaffle/tuts

-Philip