Signal analysis:
Baudline:
Baudline is a time-frequency browser designed for scientific visualization of the spectral domain. Signal analysis is performed by Fourier, correlation, and raster transforms that create colorful spectrograms with vibrant detail. Conduct test and measurement experiments with the built in function generator, or play back audio files with a multitude of effects and filters. The baudline signal analyzer combines fast digital signal processing, versatile high speed displays, and continuous capture tools for hunting down and studying elusive signal characteristics.
http://www.baudline.com/
(note: using a 10 mH inductor wired across 9v battery and connected to soundcard input)
See also :
http://baudline.blogspot.com/2006/04/vlf-sferics-tweeks-whistlers.html
Software radio:
Software radio is the technique of getting code as close to the antenna as possible. It turns radio hardware problems into software problems. The fundamental characteristic of software radio is that software defines the transmitted waveforms, and software demodulates the received waveforms. This is in contrast to most radios in which the processing is done with either analog circuitry or analog circuitry combined with digital chips. GNU Radio is a free software toolkit for building software radios.
http://www.gnu.org/software/gnuradio/doc/exploring-gnuradio.html
Wireless/802.11/WiFi:
wavemon
wavemon is a ncurses-based monitoring application for wireless network devices. It currently works under Linux with devices that are supported by the wireless extensions by Jean Tourrilhes (included in Kernel 2.4 and higher), e.g. the Lucent Orinoco cards.