Miller Puckette - the creator of PD and the co-creator of MAX/MSP
PD @ IEM - one of the main PD resouces and the host of the PD mailing list.
Chris Clepper's GEM for OS X Page
GEM @ IEM - the main page for GEM, the graphics external for PD
PD/GEM Resources and Further Downloads
Pure Data Documentation Project (PDDP) - refernce file project for standard PD objects
Yves Degoyon - lots of pd externals for audio
David McCallum/sin(theta) - a Windows PD distribution and patches/abstractions. Also some great PD made music.
PD Webring - a great list of PD links and info