Negative copier for digital camera
As digital cameras have developed enough resolution to work as scanners, such as in the scanning table proposal I wrote about earlier, some people are also using them to digitize slides. You can purchase what is called a "slide copier" which is just a simple lens and holder which goes in front of the camera to take pictures of slides. These have existed for a long time as they were used to duplicate slides in film days. However, they were not adapted for negatives since you can't readily duplicate a colour negative this way, because it is a negative and because it has an orange cast from the substrate.
There is at least one slide copier (The Opteka) which offers a negative strip holder, however that requires a bit of manual manipulation and the orange cast reduces the color gamut you will get after processing the image. Digital photography allows imaging of negatives because we can invert and colour adjust the result.
To get the product I want, we don't have too far to go. First of all, you want a negative strip holder which has wheels in the sprocket holes. Once you have placed your negative strip correctly with one wheel, a second wheel should be able to advance exactly one frame, just like the reel in the camera did when it was shooting. You may need to do some fine adjustments, but it is also satisfactory to have the image cover more than 36mm so that you don't have to be perfectly accurate, and have the software do some cropping.
Secondly, you would like it so that ideally, after you wind one frame, it triggers the shutter using a remote release. (Remote release is sadly a complex thing, with many different ways for different cameras, including wired cable releases where you just close a contact but need a proprietary connector, infrared remote controls and USB shooting. Sadly, this complexity might end up adding more to the cost than everything else, so you may have to suffer and squeeze it yourself.) As a plus, a little air bulb should be available to blow air over negatives before shooting them.
Next, you want an illuminator behind the negative or slide. For slides you want white of course. For negatives however, you would like a colour chosen to undo the effects of the orange cast, so that the gamut of light received matches the range of the camera sensors. This might be done most easily with 3 LEDs matched to camera sensors in the appropriate range of brightness.
You could also simply make a product out of this light, to be used with existing slide duplicators; that's the simplest way to do this in the small scale.
Why do all this, when a real negative scanner is not that expensive, and higher quality? Digitizing your negatives this way would be fast. Negative scanners all tend to be very slow. This approach would let you slot in a negative strip, and go wind-click-wind-click-wind-click-wind-click in just a couple of seconds, not unlike shooting fast on an old film camera. You would get quite decent scans with today's high quality DLSRs. My 5d Mark II with 21 megapixels would effectively be getting around 4000 dpi, though with bayer interpolation. If you wanted a scan for professional work or printing, you could then go back to that negative and do it on a more expensive negative scanner, cleaning it first etc.
Another solution is just to send all the negatives off to one of the services which send them to India for cheap scanning, though these tend to be at a more modest resolution. This approach would let you quickly get a catalog of your negatives.
Of course, to get a really quick catalog, another approach would be to create a grid of 3 rows of negative strip holder which could then be placed on a light table -- ideally a light table with a blueish light to compensate for the orange cast. Take a photo of the entire grid to get 12 individual photos in one shot. This will result (on the 5D) in about 1.5 megapixel versions of each negative. Not sufficient to work with but fine for screen and web use, and not too far off the basic service you get from the consumer scanning companies.
I have some of my old negatives in plastic sheets that go in binders, so I could do it directly with them, but it's work to put negatives into these and would be much easier to slide strips into a plastic holder which keeps them flat. Of course, another approach would be to simply lay the strips on the light table and put a sheet of clear plexiglass on top of them, and shoot in a dim room to avoid reflections.
It would also be useful if digital cameras or video cameras tossed in a "view colour negative" mode which did its best to show an invert of the live preview image with orange cast reverted. Then you could browse your negatives by holding them up to your camera (in macro mode) and see them in their true form, if at lower resolution. Of course you can usually figure out what's in a negative but sometimes it's not so easy and requires the loupe, and it might not in this case.