Measuring DC voltage with sound card

This cheap (starting at $1.08 incl. shipping on ebay) USB sound card can be easily modified to capture DC voltage:

 To bypass AC coupling remove C6 (if not removed it would act as a low-pass filter later) and connect 120k resistor to its pad - this would be oscilloscope/recorder input. This would result in 0-6V measurements range.

Some other cheap sound cards can't capture DC with similar modification, perhaps because of digital filtering, but this model is worth a try if you don't mind its drawbacks - single channel only (you won't find better at this price) and some DC voltage present on input (2V with 250k source resistance) that may disturb measurements. I won't replace your bench scope but may be handy with low-speed (by the rule of thumb useful measured frequency range is 0-4kHz) signal recording.

Software: http://tomeko.net/dsoundscope/C_Media2/