I have a capo, possibly the "Third Hand" mentioned by Rick, which has six individual "fingers." They are actually eccentric discs which can be rotated to an up or down position, so one can select which strings are fretted and which are open. I believe Harvey Reid (sp?) used one on some numbers (no doubt having gotten bored with being able to do so much so well with ordinary equipment). It's intriguing, and would make drop D tuning (well, actually, drop E) a very quick proposition. I haven't used it, but some day I might get to playing around with it.