Funprogramming.org has inspiring short tutorials and videos about processing. This website is ispired by funprogramming.
Processing reference is the first place to go in doubt. Processing.org has also good tutorials, such as objects by Daniel Shiffman.
OpenProcessing is an online platform for sharing and discussing Processing sketches. Here and here you can find sketches created at mehackit courses.
Nature of code is an interactive book by Daniel Shiffman. Th book focuses on the programming strategies and techniques behind computer simulations of natural systems using Processing.