Software Defined Radio
SDR is dominated by windows software, so this is going to leave all that out and deal with Linux and Mac OS software.
- NOAA weather by state - http://www.nws.noaa.gov/nwr/coverage/ccov.php
- Good scanning software - http://gqrx.dk/
- Baudline - "Baudline is a time-frequency browser designed for scientific visualization of the spectral domain."
- GNU Radio - "GNU Radio is a free & open-source software development toolkit that provides signal processing blocks to implement software radios."
- Inspectrum - "inspectrum is a tool for analysing captured signals, primarily from software-defined radio receivers."