Due to limitations of the Arduino, music is generally a no-go, However, APC have cunningly managed to get the Arduino to play audio.
The Toa Mata Band is a collection of Lego Bionicles that play various musical instruments. They play drums, synths, glockenspiels and more!
The Arduinome is an open-source clone of the Monome with easy to obtain parts. The result is similar to the Monome, but with RGB LEDs.
The Wifly Mini is the ultimate Arduino powered RC car. It's got a camera and can be controlled by console joypads and steering wheels.
This giant sphere is made up of 100's of LEDs to create a spectacular light show that changes in time with the music.
If you want a coffee to be ready for you when you get home, you can simply send a 'tweet' and the Tweet-A-Pot will get one made for you.
The Inebriator is an automated cocktail making machine. Simply choose your poison, and the Inebriator will make it up for you.
The 'Romance Pants' are clever pants that dim the lights and turn on the music when they're unzipped. Geek level 1000.
The Arduino Uno R4 boards are a massive step up in both power and connectivity.
Niryo One is an open-source robotic arm with 6-axis of movement, built for makers, educators and even small businesses.
The SoundDuino 3 offers CD quality recording and playback in a simple shield. You can finally add high quality music to your projects.