My teacher showed me a neat way of playing otherwise impossible chords. Finger a chord with you left hand. Use your right hand index finger to hold down a note higher up on the neck. Strum the chord with your right hand thumb or pinky, behind the index finger. You can add two or even three notes to chord to get really open chords that cover a lot of range, and unlike tapping the notes, the sound is the same for the left and right hand notes. You can even arpegiate the chord using the thumb and pinky.
Another technique you can try is what the LA guitar quartet call the "ninja thumb technique" where they use the LH thumb under the bottom of the neck to finger the notes on the high-e or b strings. Then they can play some strange, otherwise impossible voicings.
Ted Greene mentions a technique where he uses a LH barre but with a bent finger so the barre covers one fret on some strings and one fret lower on other strings.
Anyone else have any unusual guitar techniques to share?