A craft knife or box cutter small enough to go on a keychain that uses standard double edged razor blades.
Not I confess an original idea but something I remmber my grandfather having. His was made out of brass sheet I've used aluminium salvaged from a drink can I might buy some brass sheet for a mkii as the aluminium proved to be too flimsy but the concept seems to works as well as I remember.
I found the original so I've added a couple of pictures

Step 1: Make a Blade Blank

A razor blade is not the best thing to be bending sheet metal around, so start by cutting a razor blade sized rectangle from an old gift card/credit card or as I used a sim card holder. 43mmx22.5mm the plastic card is 0.6mm thick a razor blade only 0.1mm

Step 2: Salvage Aluminium

score around the top and bottom of a drink can and split them off. Cut the length of the resulting tube with metal shears or a heavey pair of scissors (this shortens the life of the scissors considerably so don't use the pair you normaly use for cuttig fabric)
Flatten out your sheet of aluminium note drink cans are thin and the edges ragged this will cut you just as easily as the razor blade would

Step 3: Blade Holder

I cut a rectangle of aluminium 10mm wider and 5mm longer than a razor blade. Using the blade blank I folded the long edges up at right angles. In hindsight for the next folds a second blade blank cut from the aluminium would have been a good idea but I got away with using a real blade. I next replaced the plastic blank with a blade and folded the sides over on top of it. Making sure the blade moved I moved it fully to one end of the part formed tray, placed a straight edge across the other end of the blade and folded the end up removed the staight edge and continued the fold over flat. (this last step is best done after making the outer case)After marking halfway up this edge and half way along one long edge the blade is removed and this corner cut off with scissors

Step 4: Outer Case

again I cut a rectangle of aluminium but this time 10mm bigger all round than the blade holder. As the aluminium is very thin I used the plastic blade blank to make the initial side folds. The blade holder is placed up one end of the channel with the blade down and the side folds completed making sure that the blade holder can still move. The short edge is then folded up to the end of the blade. The intention was to drill this to take a keyring but it had already become obvious that drink can aluminium sheet is too thin to be a practical material for this knife.

Step 5: Final Assembly and Use

the blade holder is slid out of the cover and one corner removed as described in the last section of step 3. The blade is returned to the holder and slipped back into the cover. fully retracted no blade should be exposed.
to use the holder is slid upto halfway out of the cover to expose a corner and length of blade. Once blunted the blade is removed from the holder rotated 180° to expose a fresh corner & edge. next time you flip it over and on the third occasion rotate it 180° again. In this way all 4 corners of the blade and it's full length are used up.

Step 6: Make It Better

I belive the concept to be sound but my choice of material poor. heavier gauge aluminium, brass sheet or flattened out copper pipe would be better. The blade holder could be plastic or wooden even. if you have access to a 3d printer.make it all in plastic.