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.

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.