GuruBlog

http://www.processing.org/ the mother of all processing sites :-)

http://www.arduino.cc/ a language and a microcontroller board that that go very well with processing

http://mobile.processing.org/ a mobile phone edition of porcessing

http://www.openvisuals.org/ a site showing how to visualize data using processing

http://www.openprocessing.org/ a site collection processing sketches

http://www.processingblogs.org/ a processing meta blog

http://www.rmx.cz/monsters/ a collection of black'n'white monsters - coded in processing (one of them has been written by me :-) )

https://www.local-guru.net/blog/tag/processing my own site

two really great art collections using processing

http://www.complexification.net/gallery/

http://abandonedart.org/

some artists using processing

http://www.flight404.com/blog/?cat=1

http://www.toxi.co.uk/

http://www.benfry.com/

http://www.groupc.net/

Books:

make things talk

visualizing data

Processing: A Programming Handbook for Visual Designers and Artists