the Doctor's sonic screwdriver. In the context of the show, it's much more than just a screwdriver: it's an artifact of a an incredibly advanced alien technology.
Technically, it's a telepathically-operated computer: you point and think. It can do almost anything. It has been seen to:
In terms of dramatic purpose, the sonic screwdriver serves as a magic wand. It serves to lend an air of mysticism to the Doctor. Because it comes from the technology of the Time Lords, it's believable as a device.
The Time Lords had been around a billion years. They'd still be around, had it not been for the unbelievable hubris of waging a Time War with the Daleks.
The technology implied by a Time War is so fantastic as to make us look like less than cave men. The sonic screwdriver works because it's something the Time Lords would have in every kitchen tool drawer.
As a systems engineer, I was always on the look-out for a good tool. What one finds is that one unscrews a lot of cases, and having a good, small screwdriver is incredibly handy. Some people resort to Leathermen. I found the Retract-A-Bit Mini.
I won't extol the virtues of this too much. Trust me, if you're a systems engineer or a technician, go buy yourself a bunch of these. They're perfect. It has the right type and size of blades for almost all daily computer work.
The pen is useless as such. However, it's perfect for depressing recessed buttons.
The square bit is the only truly useless thing about it. I have had absolutely no use for it, save annoying my daughters when they complain that it's not a "sonic" screwdriver.
It's also got a cool-factor, inasmuch as the blades are recessed. It fits into any pocket and you extend and retract the bit. It's very reminiscent of the now-9th Doctor's sonic screwdriver.
For years, I've wanted to coax just a little more functionality out of it. Specifically, I've wanted to attach a USB drive to a retractable cable attached to the Retract-A-Bit.
USB are dirt cheap -- you can get 32GB for under twenty bucks. They'e also tiny: if plugged in to the back of a machine, they're probably undetectable to a neophyte.
Now install MultiSystem. Throw in PortableApps.
Now what do you have?
A portable hard drive connected to an extremely useful tool. Sit down at any PC capable of booting from USB, and you can boot from multiple operating systems. If you run Windows, there are numerous useful tools instantly available. There's plenty of space left over for sneaker-net files.
Enter my idea for the "Stone Sonic" Screwdriver. I'm going to offer these for sale. Watch this space.