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 ALSA, ESD, 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