How about a scope based on Arduino: http://accrochages.drone.ws/en/node/90
Nice!
Although a sound card would give better resolution (16 bits vs 10) and higher sampling rate (44khz vs around 10khz for the arduino), but for relatively slow waveforms its nice to have a solution that's easy to customize. You would need to use a second board for the scope if the signal you wanted to measure was generated by an arduino, the scope board will be working flat out sampling and sending the data.