Orkney tuning (CGDGCD) is used by some of the celtic players. Strings 1 through 5 correspond with the intervals of DADGAD, transposed a fourth up. In other words, if you can play in DADGAD, Orkney should be no problem. The low C on the 6th string opens up some new possibilities for playing in the key of C or as a 4 chord when playing in G. It's said to be called Orkney tuning because CGDGCD is impossible to pronounce. I've also been messing with Joni Mitchell's "Coyote" tuning (CGDFCE). There's a lot of detailed information and transcriptions of her entire catalog on her website. Have to say, that's very generous of her.