There are a couple decent software packages available for using your soundcard as an oscilloscope. Using the line-in or mic-in will allow you to sample signals below 44.1kHz with 16-bit resolution (depending on your soundcard). For Linux, there’s an open source GTK package called xoscope that is pretty well maintained. On the PC side, you can try a package (not open source, unfortunately) called Soundcard Oscilloscope, which was written using LabView. Both programs will allow you to instantly transform your computer into a really affordable oscilloscope. Does anyone know of a good package for OS X?
Before you go probing any circuits, keep in mind that the peak voltage on your soundcard is probably less than 1 volt. For real world use, xoscope’s creator put together a buffer circuit that will take care of high voltage protection as well as a level trim that will allow you to adjust the line level to something your card can deal with. You’ll want to use this with either software package you end up using.