Arduino + Unity3d
TL;DR:
3 types of interaction: Serial, Midi, & Joystick
Serial is most basic & widely supported, but buggy
Midi needs Pro or special Bridge Tool
Joystick is simple, but one-directional
Hardware:
Arduino: Teensy (from http://skpang.co.uk/)
Linear potentiometer (Bourns PSM01-082A-103B2)
Motor: H-Bridge (SN754410 from http://www.hobbytronics.co.uk/)
LEDs (WS2812B 5050 RGB LED from http://skpang.co.uk/)
Ribbon Cable (CAB-10647, hobbytronics)