The Nature of Code by Daniel Shiffman. This site collects intermediate to advanced lessons related to math, physics, generative algorithms, and artificial intelligence.
Processing RSS Feeds by Till Nagel
TileMill for Processing by Till Nagel
3D Printing by Michael Ang
Media Processing by Davide Rocchesso and Pietro Polotti
Eclipse as a Processing Editor by Creative Coding
ANT for Processing Export from Eclipse by Creative Coding
Also, see the exciting Video Tutorials