My favourite is the "JetSlide" which I got through a German guitar specialist website. Fits with a ring on the third finger and is brought into play with a little tab pushed down by the pinky when needed. When not needed, springs out of the way for full fretting possibilities.

Trouble is that some unknown and sorry-arsed $%*&£@ pocketed it at the last blues jam I host so I'll need to buy another. Not a musicianly thing to do.

RS