There is a guy here in Austin named Sean Orr who plays a 5 string fiddle. It can go down into the viola range. He burns it up...plays western swing and irish.
The fiddle is an electric fiddle, similar to Fender's FV-1. (http://www.fender.com/products/search.php?partno=0950010281) However, it looks closer to Zeta's E-Modern violin (http://www.zetamusic.com/products/display.asp?id=63).
5 string fiddles have an extra low C string allowing the range of a viola. Many classical violin players adopt this instrument in order to avoid lugging around 2 instruments.
For example, here are some unique electric 5 string fiddles. (http://www.jordanmusic.com/V5jelcom.html)
Also, see here for strange electrics (http://www.electricviolinshop.com/electricviolins.cfm)
As far as electrics are concerned, they are easier to play and more forgiving for beginners trying to learn. However, their sound never quite matches the true depth of wood tones you get from an acoustic violin. Some expensive ones such as Zeta come very close. However, those that play these usually play them for 1) the convenience of internal pickups and for 2) playing typical styles such as jazz and fusion or even rock.
WF
---from someone who plays both types reguarly. I play a Yamaha SV-120 (http://yamaha.com/yamahavgn/CDA/ContentDetail/ModelSeriesDetail/0,,CNTID%25253D2606%252526CTID%25253D231800,00.html)