I have two that I found.
The first is probably the best, and it's all freeware. It's called "Wave Tools", and includes a spec-an, an o-scope, audio meter, etc.:
http://mda.smartelectronix.com/utilities.htm
The second not quite as well made, but the same general idea. "Oscilloscope for Windows":
http://polly.phys.msu.su/~zeld/oscill.html