Microtonal and non-Western scales derived from Bach's 12-note system are pretty interesting. I've been reading up on them and watching a few videos a bit lately. It was interesting to see how Western scales started spreading in the Victorian age due to mass production of instruments, and how odd and out of tune Western scales sound to middle eastern musicians back then, and then how traditional middle eastern scales sound odd and out of tune to people used to Western music backgrounds.
This weekend, my girlfriend's choir has a concert in sacramento with this group:
YouTube - Part 2: Matthew Grasso Papamagama
At about 0:27, you can see that he's playing a custom 7-string microtonal guitar with something like 18 drone strings for a total of 25 strings. Crazy stuff, but pretty nice to listen to.
Here's a picture of his guitar:
