If people are interested in listening to signals I'd recommend checking out the ELF to VLF bands. You can, in most cases, build a receiver that operates directly on your system's sound card. These lower frequencies largely present an antenna building challenge but there's lots of interesting things down at those frequencies. Submarines, hams, and space weather.<p>The HF to EHF bands are fun but the lower frequencies, which are mostly ignored, are just as much of a gold mine for fun projects.