A library for Linux that, when used through a wrapper script, has the amazing ability to redirect data being sent to /dev/dsp into a file. It can also act as a beam splitter and record with the audio is playing. Its native output formats are .wav and .au although in theory1 it can be piped to a coverter or encoder.
This program is extremely significant because it demonstrates that secure digital audio is impossible without co-operation by the soundcard. Any authorised decrypting player would have to decrypt the audio before sending to /dev/dsp where it could be intercepted; therefore it could only be secure if the audio could be sent to the soundcard in an encrypted state. Of course a close source operating system could prevent users from pulling vsound tricks in the first place.
I use vsound to record streaming radio shows like this:
vsound --dspout --file=cbc.wav realplay http://www.radio.cbc.ca/cbcradio28.ram
1: I haven't been able to get this to work, yet.