Slide rules were a great teaching aide, because they required you to (1) keep track of powers of 10 for yourself, and (2) understand the exponential difficulty of seeking more digits in a result.

The first point lets you guess the order of magnitude of a result before doing the detailed calculation, and that is a great way to find errors.

The second one reveals why teachers get so annoyed when 9-digit answers are given to problems with 2-digit data.

I do wish some company would start producing cheap plastic slide rules again, so my students could try them out. The online ones are not as effective as actually seeing something in the real world, squinting your eyes to guess where the cursor lies between two lines on the rule. (Hm, I can get more digits on the left end of the rule than I can on the right ...)

ThinkGeek sold a cheap plastic slide rule once. Its motion was too sticky for me to, well, stick with learning to use it. I agree there's a good product idea here if you can do it better.