Most fans of experimental string instruments are familiar with the StarrBoard, an invention of luthier, microtonalist, and mathematician John Starrett:

http://infohost.nmt.edu/~jstarret/StarrBoard.html

One of John's clients, Tim Meeks, liked his so much that he has now started a company making them, with a bit more fancy woodworking. You can check them out here:

http://www.marcodi.com/

Many musicians interested in unusual tunings and greater control and choices over the domain of pitch seek out exotic musical instruments and controllers which they hope will help them in their explorations.

Some of the most daring artists — for example Harry Partch, Ivor Darreg, Buzz Kimball and Cris Forster — have become instrument builders in order to experience the ultimate level of control and creativity with regard to their musical inspiration. This may require decades of time spent experimenting and honing skills at wood and metalworking. Experimenting with a single new tuning may take months of work to build an ensemble capable of playing in it.

Some seek out array instruments which have hundreds of keys arranged in a honeycomb matrix, able to support hundreds of pitches and large scales. There are many array designs, with many more to come. All of them are very costly to build due to the large number of keys.

Others acquire continuous pitch acoustic instruments such as the fretless bass, the violin and the trombone, requiring a well-developed ear, practice, and skill.

The Haken Continuum is an elegantly designed polyphonic keyboard with no keys. The inventor, Lippold Haken, calls it a fingerboard. It is to a normal musical keyboard what a fretless guitar is to a fretted guitar. You can play the board anywhere and you will get the pitch at the position you play, and not the pitch of some specific fret or key. Then, if you rock or wiggle your finger as you would on a guitar or violin string, you have control over vibrato. The Continuum is also sensitive to polyphonic pressure, and it is also sensitive to motion of each finger along the x and y axis. This instrument has all the expressiveness of a violin. Like a violin, mastering it takes dedication, but the pay off is an expressivity which you can attain through no other means. Because of the complexity of the design and its cutting edge technology, the Continuum is as expensive as a finely crafted guitar. You get what you pay for — this is a beautiful and powerful instrument, each one individually hand made.

Here are some musical examples of it being played.