alex listed "pd-extended" as one of the well-known forks. but what constitutes the forkiness in this case? apparently not the large number of externals shipped (as this would make it as "distro"), but something else. from the top of my head (as I never was a Pd-extended user myself), two things come to my mind: tooltips and white object-backgrounds (going along with those red signal connections).
Alex certainly knows more about this, but here are a couple of things that I remember:
revamped help patches (standardized structure + hyperlinks)
different styles for audio/control inlets/outlets
inlets/outlets highlighted on hovering
different/better font size handling
the [import] object
the libdir system (*-meta.pd files), although I don't know how it
worked in practice
different dragging behavior in graphical arrays
...
Christof