Complexity Explorables - a most lovely library of web-simulations on complexity theory and dynamical systems (percolation, pandemics, flocking, spin systems, reaction-diffusion, condensation etc.)
Recursive drawing - How a set of really simple rules can create really complex structures
Geometriize - AI painter; a good illustration of how art works (as an AI algorithm, it captures some aspects if art, but also totally misses some that a human painter would never miss. Good for reflection on the nature of art.)
The evolution of trust - a simulation tutorial on game theory, and on why biologically you would not expect all people to be nice to each other
Web-based textbook on Neuroscience from U. Texas medical School - This book is more advanced, and generally approaches the Neuro1 level. Some parts are actually simpler than I would prefer them to be, while some sections are slightly overloaded with details, but overall it's pretty nice. And for a free book it is just unbelivably great! Also it has some nice flash-based interactive illustrations for every topic.
Neuroscience by Dale Purves - this is one of the classical "Heavy books" on the subject, and it is available for free, except that it's an old edition (from 2001), and you cannot browse it page by page. Instead you have to use the "Search" function they have. But it's quite intuitive, really: just put there a good search term (such as LTP, claustrum, or kainate), and read the chapters that matter.
The dimensions of color - a textbook for painters (not neuroscientists of physicists), but if you are interested in the science of visual arts, you should really read it as well.
Auditory neuroscience - lots of useful videos and sound files that illustrate how our auditory processing works (including auditory illusions, cortical implant simulation and alike)
JOVE is a Journal Of Visualized Experiments, and most of its materials require a subscription, but some videos are open-access. Here's a list of some experiments I used in my course; some of them may be free, some may require a subscription.