If you are a windows user, Christian Zeitnitz offers a PC based Soundcard Oscilloscope free for non-commercial use. It also has a frequency spectrum waterfall diagram, x-y plots. Easy to install and run. Fun to speak into your microphone to test it out.
Only suitable audio speed signals like XOScope. I.e., 20-20000Hz from 44.1kHz sampling and 16-bit resolution. And without external hardware voltage dividers/protection the usual warnings about blowing up your soundcard if you feed in voltage outside of ±0.7V into it.