Hacking festivals to me is synonymous to two things: The copious amount of generic hacks that comes in all shapes and forms, but also the interesting ones that wows you the instant you see it. Such as this Beat Bricks program. Beat Bricks is a step sequencer that tells your music software (Ableton Live in this case) the kind of beat pattern it should perform.
So how does this work? Well I have yet to take a look at the implementation and this is just the computer-science in me that speaks: the whole thing themselves seem to consist of LEGO bricks plus the pads to put the bricks on, and a webcam that is connected to your computer. This webcam keeps on watching the formation of your LEGO bricks and tells the software inside your computer about the beat it's supposed to make. The green pad could act as a "cue" to tell the software that feeds on the image recorded from the webcam about the area where the LEGO would be placed on, besides of course providing firm holds to the bricks. Without this pad I imagine that the software could have a hard time calculating the relative position of the bricks. It also appears that bricks of different colors does different thing, as demonstrated in the video that some bricks trigger melodies instead of beats. Definitely some nifty image-processing works.
Granted it probably doesn't offer you the features found in your favourite software step sequencer or your drum machine if you're lucky to have one. Even so, it's still amazing to see such interactions between your software and things that exist in the real world.
For you curious minds out there hold on, unfortunately no easy to configure executable is provided. Those familiar with programming could try their luck by pulling the python source code and run it on their machine. Good luck!