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.