Visible, physical, simple math... and magical when scrawny little me can lift Big Thing.
Hydraulics / Pneumatics are not as old and aren't as open to inspection while operating, usually; so the magic seems stronger there.
Today's digital systems are still built atop quantization of analog effects; the edge cases and errors (and the techniques used to avoid and correct those errors) might actually be magic, if you want to get into metaphysical wanking about definitions. The lovely maths of signalling systems even resemble those of pulleys, some ways: it's the same magic.
A friend of mine invited me to an art installation made by her and 20+ of her friends which was basically a low-budget version of an attraction at Disney. (You go into a "time machine" and visit different places and times by opening the door it tells you to open)
One of many fun features was a sand table that had a first-generation Kinect and a projector that projected contour lines on the sand that shift when you change the surface. It seemed pretty magical, like AR turned inside out.
It got me thinking a lot about things you can do with projectors, which Disney has been doing a long time. (For instance, Disney puts a Star Wars theme and other themes on Space Mountain by mostly changing audio and video so they can retheme it for special occasions such as when a Star Wars movie comes out.)
I recently found myself in a situation where I had to use a Window 7 machine for a couple days. I nearly cried. I'd forgotten what it used to be like to experience joy (and not endless frustration) when computing.
The fact I can talk on the phone to someone thousands of miles away feels pretty magical to me when I think about all that goes into it.
Other tech that feels magical to me:
* The browser: wait, I can publish something and it is visible and interactive to anyone with an internet connection? And all I need is a text editor?
* Steel: we can build huge structures hundreds of feet high, drill thousands of feet under the earth, all with metals we refine from rocks?
* Matches: I can, for a very little amount of money, get a box of matches which will let me start a fire with close to zero effort?
* Hardware Video Editors (Genlocks, Toasters, Non-Linear Editing Consoles)
* 3D graphics from Evans and Sutherland to the golden days of SGI days, long before there were any standards one could build upon.
* The Nintendo 64. Enough Said.
Despite being worked on for centuries, optics are still a super active field for fundamental research, R&D, and new products.
LEDs are incredible for example. Extraordinary efficiency.