Bandwagon - This group has recently done some interesting demos for the MSX-1. - http://gr.ath.cx/bandwagon/
Pear Software - So far they've only released a demodisk and a Moonblaster musicdisk. This one was also released for PC. - http://www.xs4all.nl/~scrinium/pear
Lieves!Tuore - Dedicated to the 1st generation MSX, they've created some impressive demos for this machine. - http://www.kameli.net/lt/