I was recently shown "dropped D" tuning, where the lowest bass string is tuned to D instead of E, but everything else remains standard. It gives a really cool dark tone to stuff in the key of D. It uses some standard chords and some alternate chords that are the same or similar to DADGAD, which I also just discovered and is also a really great sound. My hands are very small and one finger is shorter than it used to be, so some of the chords are really physically impossible for me, but others are very simple. I am an absolute novice with this and have only messed around in D, but it's great fun.