What follows is a curated list of Software oscilloscopes. The idea is always that you can connect your Analog Thing to your Soundcard input with a simple Chinch-to-jack adapter, such as depicted in the right. This allows to connect to the 3,5mm phone connector (Klinke) which is still present in many Notebooks, Tablets, Smartphones, also for many system-on-chips such as the Raspberry Pi. If your system does no more have such an input, there may be cheap adapters to USB (they are usually called USB soundcard).
Note that a single chinch socket has to pins: One for ground, one for the signal. This puts it on equal footing with a BNC connector or a mono phone connector. The adapter shown on the right joins two outputs together to a stereo sound signal.
Web based oscilloscopes
You can use https://anabrid.dev/webosci/ from any web modern browser. It will ask you for recording permission from your sound card. Make sure you connect to the proper input and not to your webcam microphone, for instance.
Known limitations: Older versions of Safari, Firefox and Chrome do not support the relevant Web standards. In such a case, you should try out some other attempt.