This is a Pocket Operator. 

It's a miniature synthesizer that can store 136 sounds and sequence them across 16 pattern chains.

This one is the PO-33 KO, one of the series with a built-in sampler. That means you can pull in audio from anywhere and use it in your sequencing.

It normally looks something like the skeleton of a calculator, but I custom-printed a case using a few blueprints from Thingverse.

This page contains some of the songs and snippets I've created using my Pocket Operator.