I forget which youtube personality mentioned it first, but there is an arduino sketch and supporting software to create a crude oscilloscope (see information, below). The "probes" are just jumper wires, and the "scope" is pretty limited, but it was fascinating to actually see waveforms and I was able to debug a memory timing issue using the arduino as an oscilloscope.

Serial Oscilloscope

You can find more information, and downloads, at x-io.co.uk/serial-oscilloscope/.