#1
I love alternate tunings, I've been messing around in Eb, open g, open d, drop d, & dadgad, looking to expand my horizons. I'm into hard and blues btw.
#2
That's the limit of my interests, but how about partial capoes? For example, I play slide, and have one that does 111011 on open D, I actually use it as 222122 with a full capo to make open Em. Good for tunes that start on a strong minor, then mainly use major chords, like "Hotel california".

Harvey Reid does a lot of stuff with partial capoes:

http://www.partialcapo.com/partial_capo_basics.htm
#3
C G C F A E is one that I "discovered" one day and really fell in love with. A few of my pieces are in it.
My God, it's full of stars!