Using a soundcard als low-cost oscilloscope is an idea that may be as old as soundcards themself and there is plenty of software with oscilloscope-like user interfaces. (Just google for sound card oscilloscope). One of the main drawbacks is the AC coupling of soundcards. To measure DC voltages, the soundcard must be modified (coupling capacitor removed) or an additional external circuit must be used. This page presents such a circuit which is extremely cheap and simple.
The software is a Java program which uses JChart2D for visualization. Just download the binary zip file below and run SocDVM.jar. A more detailed description is included within the program (Help => About).
The software displays the measured voltage numerically and in a "scope-like" view. The scaling of the axes is configurable and the measurement results can be either saved as image or text file to allow futher processing of the results with other tools.
There are two different hardware versions available. The older one is the voltage to frequency converter hardware which has some issue concerning temperature drift, but is extremely simple.
The newer hardware is a complete delta sigma analog to digital converter which is less prone to drifts but more complicated.
Read more about the required hardware and the schematic on the hardware subpages linked in the above paragraph or directly skip to the download of the schematic.