The Asahi Mirror Adapter II
A must in any self respecting photographers bag. How else are you supposed to take pictures around corners. Screw it onto your long ranger and everyone will think your taking a picture of your neighbours wife in the shower when actually your bagging that Waxwing in Tesco's car park. False glass on the front and a 45deg mirror in the side. You have to use something longer than 50mm as you get shadow cast around it pretty badly. It works on my 135mm. Haven't actually used it in anger........ honest.
The Pentax Stereo Adapter II and Stereo Viewer II
Long before the days of Avatar there was the stereoscope. You have two images side by side taken a few inches apart. When you look at them through the stereo viewer you get..... wait for it....... 3D. Yes the word comes alive and IN YOUR FACE! It's amazing. Unfortunately you can only see slides in the viewer supplied. If you want to see the results on your screen you have to use the cross eye method. Cross your eyes until the two images combine. Looks cool on the end of your nifty fifty.