I have a Sansa Clip+. It's an MP3 player. Remember those?
Physically, it is about the size of a matchbox. Not the tiny model cars, which were always too big for their namesakes, but the actual give-away advertising matchboxes, made of sturdy cardboard with a friction strip on one side. It's a little larger than that because of the spring-loaded clip on the back. I didn't realize how much the clip contributed to the usefulness of the device until it broke. Turns out that without the clip, you have to find a pocket or hold it in your hand. Not good.
It has extremely standard ports. It has a USB mini-B port, on which it can be recharged (14-15 hours of playback per charge, not too shabby) or you can manipulate the data it stores. It uses the USB mass-storage protocol, so Windows, Macs, Linux and random other machines can read and write to it easily.
There's a 1/8" stereo headphone jack. That's the linear descendant of the 19th century switchboard operator's plug, entirely recognizable and usable by a technologically advanced person from 1878. Wikipedia says it's the oldest electrical plug standard still in use.
The side with the USB port has physical volume buttons, up and down. The side with the headphone jack has a slot for a MicroSD card. Officially, it's only good up to 32 GB. In actual fact, a 128GB MicroSDHC (not SDXC) card will work quite nicely.
On top there is a power button; hold it down long enough not to be deemed accidental, and the machine will change power states.
The front face has a tiny display and six buttons. 4 arranged at cardinal points, one in the center (which means Yes, or OK) and one off to the side which changes modes. Because it is slightly assymmetric, your fingers can find the buttons and puzzle out which one does what without needing your eyes to see it. (And it's not a touchscreen.)
If you leave it with the original firmware, there are several annoying issues. First, it can take many seconds or even minutes to boot up. Second, it plays everything 1% too fast, and thus at 1% too high a pitch. Third, it's not very customizable. Installing the Rockbox firmware solves all three of those problems.
So: it's small, does one job really well, lasts a long time, has a high capacity, and is customizable. All of this while being standards-compliant in three different interfaces. In a sense, this is a peak expression of my technical philosophy: do the things that are known to work well; put them together in ways that make sense. Innovate where you need to, and not where you don't.
Clearly I love it, and other people do too: used Sansa Clip+ players sell for as much or more than the original list price.