OK this just went on the list of things to do. The author took a $15 USB sound card, some resistors, wire, and jacks, and made a simple Oscilloscope out of it. I have seen folks do this with other soundcards in the past, but not this way (with USB cheapy). And I have seen the arduino way to, but I like this one since you can get a feed into a PC to record and perhaps grow this into a logic analyzer.
http://www.instructables.com/id/USB-Oscilloscope-with-Signal-generator/
Update, I did a quick search on ebay and found similar cards for under $5.