^ You beat me to the punch, because I was gonna pull out the Andy Summer's chord. I adapted it to drop-d on my own, before I knew other people did it:
It adds the octave above the fifth to the mix. It is a very full, piano-like chord.
Here is another favorite of mine, which I use when I'm in drop-d, also very piano-like:
x55578x movable form
x555785 add9 form
x000235 open chord
It's just a whole bunch of roots and fifths, but when you spread it across three octaves, it sounds really full. Then you can always drop the ninth at the end for a little tension. You can take the open form, and only using the treble strings, play a power chord voicing much higher. This sounds awesome beneath another guitar playing power chords on the bass string.
Not a ninth, but this is a cool, oddball form that I occasionally work into a riff:
It's a third and an octave, and has this awesome "clonk" sort of sound to it. They make awesome passing chords. I've only ever used the major, since I don't have the hands to pull off the minor form, but I bet it sounds cool.