A while ago my daughter asked me to show her how beat-matching is done. Unfortunately we had no DJing equipment and even the entry-level MIDI controllers on Amazon are quite expensive. After some googling we've discovered projects like Fliper DJ and Traktorino, but the former wasn't quite what I've wanted and the later wasn't available at the time, so we've decided to build one ourselves.
Few knobs, buttons, wires, Arduino Uno (+ CD74HC4067 multiplexer) and here we go:
Project repository on GitHub contains the code and Traktor 2 mappings.