I've started looking into software defined radio for Mac OS X. The goal is to take a cheap USB stick, plug it in to the Mac, and have it connect to software that will decode radio signals.
So far there's nothing obvious that's packaged as a single collection of software plus hardware, but that's not surprising.
My best notes to date are from Pinboard's t:sdr tag, which looks like it has a few dozen people sharing links.
I'll update this when I get more working details.