The Bliptronic 5000 is a great and super-cheap rival to the $500 Monome although not with such a great chassis and quality. But: it is chainable and has its own built-in sound engine (chiptunes-style, sound similar to the tenori-on or the casiotone)
createdigitalmusic posted some hacking tips, including information on how to build your own MIDI clocking and how to create a "switchonome". Another post by runagate describes how to Turn A $50 Bliptronic 5000 Into A Monome Controller [update: Bliptronome conversion kit for $68 USD].
Information via ThinkGeek:
* Unusual retro synthesizer is played with a grid of glowing buttons
* Create looping patterns and change them dynamically while playing
* Chain multiple units together and create more complex melodies
* One octave range. 8 notes can be played simultaneously
* 8 different old-skool synth sounding instruments to choose from
* Sounds created using FM waveform synthesis
* Set the BPM (beats per minute) from 60 to 180 in 20 BPM increments
* Built in speaker with headphone jack and line-out jack
* Front panel is constructed from brushed aluminum
* Includes, manual and 2 link cables for connecting additional Bliptronic units
* Requires 4 x AA batteries (not included)
via weburbanist: "The study of musical instruments (’organology’ – no, really) is the study of the human condition. Every culture is defined by its own distinctive set of trills, whistles, parps, honks and beats, and every corner of the world has evolved its own location-specific indigenous instrument to renew a sense of cultural identity through noisy self-expression. And instruments evolve – never more so than now, in the midst of a technological revolution that has opened up entirely new ways to make music. So settle back and compose yourself as we look at ten new instruments that look set to accompany us into the world of tomorrow."