Rollei made a number of cool gadgets and accessories for their Rolleiflex TLR cameras, but we think our favorite is the Rolleimeter. This nifty device provides your Rolleiflex with rangefinder focusing. You attach it to the front of the hood of the camera, calibrate it for both close focus and infinity, and then pop open the direct "sportsfinder" viewfinder and away you go.
One final note, this is the 3.5 version. The Rolleimeters had a little plate you swapped out for use with 3.5 or 2.8 cameras (from our understanding of the device) and this one has the 3.5 plate. It will work on all the Automat cameras and up to the 3.5E. But sadly it does not fit onto a Rolleicord or the Rolleiflex T.