Peter Tsiorba mentioned during a phone conversation a couple months ago that seven-string guitars (known as Russian guitars) are popular in former Soviet Union countries. I think he said that his mother used to play a seven-string guitar. He can correct me if I remembered that wrong. Anyway, we were talking about seven-string guitars, because I had just watched a TV program about a young guy of Ukrainian-Gypsy decent who had a band that was playing Gypsy-punk fusion music in New York City. The young guy took recordings of his Gypsy-punk fusions to the Ukraine and played them to Gypsies who had about the same reaction most of us have to flamenco-jazz fusions.
However, he eventually visited a Gypsy musician who was more sympathetic and said that he understood that was a way to gradually introduce Americans to Gypsy music so they would eventually come to like the real thing. That Gypsy taught the young guy to play and sing a Ukrainian-Gypsy song. The older guy had a seven-string guitar that he explained (in Gypsy) was tuned open to an E-minor chord. The younger guy, who had a six-string guitar, initially misunderstood, and started playing in E-major.
This is a short audio clip of that encounter that I thought you might find interesting in connection with Sam's seven-string guitar post above. It starts with the older Gypsy letting the young guy hear the sound of his seven-string guitar and then proceeds to teaching the young guy to play and sing the song.
Ukrainian Gypsy 7-String Guitar.mp3
It is my understanding that a variety of different seven-string Russian guitar tunings are used, but that the top six strings generally are not tuned like a six-string guitar as they were in the YouTube video in the post above.
The 7th top string is usually tuned to C and the rest E,A,D,G,B,E for Brazilian, but many playing flamenco tune it to B. I would think Bb might be interesting. Lots of tunings for a 7 string. http://en.wikipedia.org/wiki/Seven-string_guitar