Musical Instrument Digital Interface - the standard that has been with us forever when it comes to music. Sure, we don't use those nasty 5 pin DIN plugs anymore (thank goodness) but USB host ports and MIDI are still the way to go if you want to do any serious music production.
Most of the time I can't access my piano because it happens to be in the living room of a house in the wrong country, but when I can get to it I'm lucky enough to have access to this beasty.
I have the V1 but this is still a nice piece of equipment. I'm by no means a piano player but in keeping with liking things that I can mess around with, this digital audio workstation has a whole bunch of things you can mess around with. It has an auto accompaniment section so you can sound like you know what you're doing, without actually knowing what you're doing. It has a USB To-Host port on the back so you can jack it into your computer and use it as a very expensive MIDI controller. The keyboard quality is top notch and can get just about anything done.
It doesn't actually matter what I use to author any piece of music but notably I used this keyboard in Building Our Dreams Today. Not that you can tell. I'm going to post a link to it here though because I can.
It's a song. I wrote it with my piano. Whoopdeedoo.
There. I feel better now.