Jean and Lee Schilling's capo let you choose which strings to capo by rotating a rubber disk either down to the string, or up to leave the string free. This design was inspired by the "extra hand" guitar capo.
This is a capo that I use a lot, instead of retuning. I play a four-course instrument tuned CGBc. The B-string gives me the missing chromatic notes, and with the Shillings' capo I can capo the open B to c while still keeping all those chromatic B-string notes (this trick requires an extra fret to play the half-step between the nut and the traditional first fret).
Here's a photo of the Schillings' capo: