Gizmo is the codename for an open source MIDI utility device which targets the Arduino Uno or Mega. It can do at least this:
Applications
Full-featured Arpeggiator
Full-featured General-Purpose Step Sequencer
Full-featured Drum Machine-style Step Sequencer
Small Note Recorder
Capable MIDI Gauge or Monitor
Small two-button, two-pot MIDI Controller
8-stage Loopable Control Envelope and Random/S&H LFO
Keyboard Splitter
Thru facility with Channel Merging, Chord Memory, Debouncing, Multichannel Note Distribution, Channel Blocking, and Note Replication
Measure and Beat Counter
MIDI NRPN/CC/Sysex Converters for Blofeld, Matrix 1000, Microsampler, and TX81Z
Capabilities
MIDI In and Out
NRPN, CC, and by-Note Control of Gizmo
Swing
Bypass
Additional control via joysticks and other pots
Tempo and Note Speed
MIDI Clock Control, Filtering, and Division
Click Tracks
Transposition and Volume Modulation
Storage for 10 Arpeggios and 2 (Uno) or 9 (Mega) Sequences / Recordings
The Uno can contain a single application from the above list, except the Drum Sequencer (and the Step Sequencer is limited in ability). The Mega can hold all of them at once (get a Mega!). I have not tested Gizmo's capabilities on other Arduino platforms, though I know that other people have.
Other Projects
Flow, a fully-modular, polyphonic, additive software synthesizer.
Edisyn, a patch editor toolkit with sophisticated exploration tools.
Gizmo's LED display is rock-solid, steady bright red. But in these videos it looks like flickery, junky yellow due to the camera. :-( Annoying. Note: these demos were of Version 1: there have been improvements since.
Gizmo relies on the esteemed Forty Seven Effects MIDI Library, though you should install the version of the library which comes in Gizmo's repository.
Assembling Gizmo
To assemble Gizmo, you just need an Arduino Uno or Mega (if you want to develop, or have all of Gizmo available at one time, definitely get a Mega), a SparkFun MIDI Shield, an Adafruit 16x8 LED Matrix Backpack (they come in various colors, shapes, and sizes -- I use 1.2" round amber red; red, light green, and orange seem to make the least noise), and some wire. And you'll do a bit of soldering.
Enclosures
John Pierce build a beautiful enclosure for Gizmo in wood. Click on the pictures below for larger versions
Damian Raistrick wrote in with pictures of his elegant custom enclosure for Gizmo. Check it out!
GearSlutz user Saint Gillis passed along his enclosure too!
Simon Davis shows off a gorgeous picture of his acrylic enclosure:
And now for the coup de grâce. GearSlutz user PatMaximum has provided photographs of his Five-Gizmo monster. They're all headless versions controlled remotely over MIDI.
Development Ideas
I'm making Gizmo available in the hopes that people will do fun things with it. Ideas I've had include porting it to the E-licktronic MIDI shield family, extending the interface to include the 16x8 Unztrument or even just the Trellis Keypad, upgrading to an OLED (though it may be too slow), attaching an arduino synthesizer shield, and of course making a decent case for this thing.
If you're thinking of doing development for the Gizmo, you should do it on an Arduino Mega. At present Gizmo takes up all but just a few bytes of the Arduino Uno's memory: there's no space left to code.