Quasimidi is a German Synthesizer manufacturer.
They've become famous with their Rave-o-lution 309, a virtual-analog hybrid between the Roland TB-303 Bassline and the TR-909 Drumbox.
Other Quasimidi products include

• the Quasimidi Sirius : a Virtual Analog keyboard including a vocoder and an automatic tempo-adjusting unit.
• the Quasimidi Polymorph, which features an Analog Sequencer interface
• the Quasimidi Raven, an impressive techno synthesizer

Famous Quasimidi users are Jean-Michel Jarre, Klaus Schultze, Scooter and Kraftwerk, who are highly suspected to help Quasimidi's engineers while creating their units' excellent presets.
For more information, please visit : http://www.quasimidi.com (s'gibt auch eine Deutsche Version)
Update: 2001: QuasiMIDI is dead. Their website is closed.

In addition to mirko's writeup:

Unfortunatelly Quasimidi are no longer in business and their websites only contain an error message in German (for the .de site) and a place holder porn ad (for the .com site)

Quasimidi's products often got a mixed reception when they were launched: you either hated or loved them. They went their own way with just about everything (with the possible exception of the rave-o-lution) and some of the amazing features was almost impossible to get to work due to the less than intuitive user interfaces.

If you want something that doesn't sound exactly like the Rolands, Korgs and Yamahas everybody else use, you might still want to try out a Quasimidi synth if you get your hands on it. Personally, I use their Quasimidi Cyber 6 midi controller keyboard (basically a Raven without the sound generators) and love it!