xoscope is a digital real-time oscilloscope.  It graphically displays
signal amplitude or bit logic as a function of time.  Signals may be
displayed, saved, recalled, and manipulated by math functions.

Signal input devices currently include /dev/dsp, EsounD,
ProbeScope/OsziFOX, Bitscope, and COMEDI.

If you use your sound card as the input, consider building this buffer
circuit to protect it from overvoltage:
http://xoscope.sourceforge.net/hardware/hardware.html