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