I've used GNU Radio and built a few different virtual radios. I finally grok the idea of a negative frequency (which only makes sense if you have both I and Q)
I wrote a flowgraph that decodes the local VOR Navaid, and shows my bearing to it. GNU Radio is also suitable for audio I/O by the way.
At some point in the future, I'd like to set up a coherent SDR with many channels, so I can do passive radar and see what's really in the sky.
I'd also like to do my own GPS from scratch, just to know that I could.
I've got a RTL SDR on order, and I plan on using it with SDR#. Mainly I will be exploring the radio airwaves, seeing what's out there. I just wish there was an inexpensive SDR that would also transmit, especially for amateur radio frequencies.
I looked at several radio microphones, which were used in different bars in a building with local sound systems. Some of the microphones had been mixed up between rooms and some were entirely lost.
There is some software called Spektrum which I found handy for looking at the full range up to 1GHz to see roughly where the microphones where transmitting then zooming in.
Writing down a list of the microphone frequencies and the frequencies on the labels on the receivers let me figure out what was going on with less running around.