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 $68 USD].
Information via ThinkGeek:
* Unusual retro synthesizer is played with a grid of glowing buttons
* Create looping patterns and change them dynamically while playing
* Chain multiple units together and create more complex melodies
* One octave range. 8 notes can be played simultaneously
* 8 different old-skool synth sounding instruments to choose from
* Sounds created using FM waveform synthesis
* Set the BPM (beats per minute) from 60 to 180 in 20 BPM increments
* Built in speaker with headphone jack and line-out jack
* Front panel is constructed from brushed aluminum
* Includes, manual and 2 link cables for connecting additional Bliptronic units
* Requires 4 x AA batteries (not included)
Monome.org just released the information in the release date of their greyscale sixty-four: 1st of January, 2010. The 64 edition is US$ 500 plus shipping costs. *Update*: Orders are accepted as of NOW. I just ordered my monome via google checkout. International postage fee is 90$ +customs. Let's see when it arrives...
What is a monome? "a reconfigurable grid of backlit keypads which connects to a computer. interaction between the keys and lights is determined by the application running on the computer. there is no hard-wired functionality."