There have also been things like bass banjos and bass balalaikas posted here. I don't know if anyone is actively playing them.
Probably the oddest thing I've got is a three-necked six-string. That's six strings total, you understand... one neck with three strings, one with two, and one with one It's technically a bass guitar, I guess. I don't really know what it's for; my best guess is to accompany several trad instruments where the natural frequency of the trad instruments varies from instrument to instrument, and retuning every time was not sensible. But really, I don't know.
Obviously I bought it here