An isomorphic (or self-transposing) keyboard is "a musical device where a grid of notes is displayed to the user and the interval change between notes in constant for any given direction." For those of you who didn't grow up playing the piano like I did, that basically means that it lets you move between keys easily without having to learn new patterns because the keys are laid out so that a chord is the same pattern in every musical key.
It's much easier to learn to play on an isomorphic keyboard, and for songwriters, it makes transposing a cinch. That alone makes it a pretty nifty tool to have around, and it doesn't hurt that this DIY version made by Brett Park of Shiverware Interactive is absolutely gorgeous.
He calls it the Rainboard (for obvious reasons), and he built it using an Arduino Uno Board, a bunch of RGB LEDs, and some clear acrylic. After drawing up the design in CAD, Brett cut out the acrylic, punched 61 holes in it, and wired an arcade button and LED in each one.
He coded the Arduino and assigned a midi value to each key, using his Musix iOS app to control it on his iPad. He also added a touch strip to the side as a pitch bend.
I've never played on an isomorphic keyboard myself, so I'd be really interested to hear about it if anyone decides to tackle this project for themselves. You can find a detailed parts list, step-by-step photos and instructions, and all the code over on Brett's blog.