I bought a second-hand USB oscilloscope from the internet, and it
arrived today. I believe the model is Hantek 6022BL. I was a bit
worried about Linux support, but thankfully some people already
reverse-engineered the device and wrote Linux userspace drivers for it.

I didn't have time to dive too deeply into it, but I played around a
little bit. Here are some tiny projects I've done with it.

1. Figuring out the mains AC frequency.

  I wrapped a random alligator clip cable around my extension cord,
  probed one end of the cable and observed an oscillating signal,
  precisely at 50 Hz (which is the AC frequency in my area).

2. Audio audio card

  I probed an audio cable, and used the Python API to capture samples.
  I then piped those samples to `aplay` in order to play them through
  my computer's speakers.

  This is an interesting way to use the oscilloscope as a high
  sample-rate audio card. And due to having two channels, it can be
  used for stereo recording as well.

The graphical oscilloscope application I used on Linux is called
OpenHantek6022. And the Python API and open source firmware is called
Hantek6022API. You can find the links to those below.

=> https://github.com/OpenHantek/OpenHantek6022
=> https://github.com/Ho-Ro/Hantek6022API