The PD community page:
http://pure-data.iem.at

PD @ Sourceforge (CVS etc.):
http://www.sf.net/projects/pure-data
http://pure-data.sf.net

Miller Puckette's PD source:
http://www-crca.ucsd.edu/~msp/software.html

Max/MSP/Jitter:
http://www.cycling74.com

Links for cross-platform PD/Max external programming:
http://www.cvsgui.org 
http://iem.kug.ac.at/pd/externals-HOWTO