I've just started dabbling in TensorFlow, which is an open source library for building Neural Networks (as well as other high-powered computational stuff).
Neural Networks are something I learned about years ago, but of course back then it was mostly mathematical theory - we didn't really have the tools to see them work in real time.
The funnest site I've found in a long time belongs to Tensor. It's got this amazing Neural Network Playground where you can screw around with Neurons and watch them try to figure out how to recognize an image.
You don't need to know anything to get started. Just click the big round play button at the top, and without even choosing any particular settings you can watch it go.
I strongly suggest checking this out. I've wasted literally HOURS playing with it...