I really have no excuse for not introducing the Teenage Engineering OP-1 until now. It first gained fame being featured in the swedish house Mafia's music video for "One (Your Name)". It's a quirky portable music workstation.
Here's one example of the OP-1 in action:
And here's a tutorial to the ridiculous and wide accessories range, which makes the OP-1 even more a real Thingstrument.
The unit currently costs 800€, so it's not the cheapest toy you can get, but it can really do quite a lot for the buck.