My colleague Hannes uploaded a simple Multi Layer Perceptron as a demo for our CUV library.
It is written entirely in Python and classifies the MNIST dataset of handwritten digits. He also wrote a blog post explaining the design and use in detail.
The code is very easy to understand and to expand.
But it is very fast - of course, using CUDA - and can serve as the basis for many experiments.
There are also convolution routines in the library and its even possible to extend the network to a convolutional neural network entirely in Python and on the GPU.