There are at least two models that I'm aware of: Sam Rizzetta's Piano Dulcimer, and James Jones' Linear Chromatic. If memory serves, the Piano Dulcimer has its half-tone steps running across the bridges, while the Linear Chromatic has them running from string to string, and fifth intervals across the bridges like a standard dulcimer.
Although the value of these instruments has yet to be demonstrated, I feel it could be great. However, attempting to play either of them once you're accustomed to a standard dulcimer could be difficult, like learning a whole new instrument.
If you haven't played any kind of dulcimer, you might consider one of these models if you plan to play complex music in a wide variety of keys. On the other hand, if you're more interested in traditional folk music with occasional forays into more complex pieces, then a standard dulcimer may be the one for you. Just remember that, right now, teachers and instructional materials are scarce for the Piano Dulcimer and Linear Chromatic. If you buy one of these models, you'll be a pioneer.
For more information on the Piano Dulcimer, visit Sam Rizzetta's web site at hometown.aol.com/rizzetta or Dusty Strings at www.dustystrings.com.
For more information on the Linear Chromatic, visit James Jones' web site at www.jamesjonesinstruments.com/linearchromatic.html. James offers the Linear Chromatic in two sizes, and offers a student version of the smaller one at a very reasonable price.