TFD Scope is a small application that allows you graph any signal read from the input of the sound card in real time. You can read signals with a microphone connected to the sound card, but since the input of the sound card has a left and right channel, you can read different signals, one signal per channel (the signals connected to the input have to be low voltage signals otherwise you can harm the sound card of your computer).
The signals are shown in two windows. The upper window shows the signals in a time-domain graph as an oscilloscope, and the lower window shows the signals in a frequency-domain graph. To begin watching the signals you just have push the "START" button. By default both channels are graphed (1ch and 2ch), but you can click on each channel to show or hide the signal. In both windows you can amplify or reduce the signals with the "Y-amplitude" slide bar. The "X-time" slide bar allows you to change the time base. The "X-frequency" slide bar allows you to change the base frequency.
The only disadvantage I found is that the signals graphed cannot be recorded or saved to file for further analysis. You cannot take snapshots of the graphed signals either.