How about this: an 8-string violin -- http://www.fiddlehangout.com/classified/1850 (oops, spelled "mandolinist" wrong in the headline . . . can't correct it.)
very, very, very cool ... wonder how difficult it is to keep in tune, however.
It has mandolin-style geared tuning machines, so getting it in tune is probably not that hard. But as you say, will it stay in tune?
Interesting - Can't do much with double-stops, I'd guess.
That is interesting. I'd rather have those "extra" strings under the fingerboard, though.
