Virtual Audio Cable is an important software tool for the Windows rtl-sdr user. It allows an audio data stream to be piped from a software radio program such as SDRSharp, into a decoding program. Unfortunately Virtual Audio Cable is not free, which is not in the cheap spirit of rtl-sdr, and at $25 it costs more than the dongle itself. There is a trial version, but it periodically plays a watermark voice which can break decoding.
A free (donationware) alternative to Virtual Audio Cable that we recently found is VB-Cable. VB-Cable does exactly the same job as Virtual Audio Cable but for free. By donating any amount to the developer, you can unlock two more virtual audio devices, which is useful if you use multiple rtl-sdr dongles on one machine.
Note, one trick to installing VB-Cable is that I found that I had to run the installer in Administrator Mode on a Windows 7 machine to get it to work.