Software
General
gqrx - A user interface to tune and listen across a wide range of frequencies
sdr++ - Better alternative
dump1090 - Listen to aircraft 1090MHz transmissions and display on map
dump1090-mutability --write-json /var/spool/dump1090 --interactive
gpredict - satellite location
qsstv - decoding SlowScan images (eg from the ISS)
noaa-apt - weather saterlite image processing
rtl_433 - Listens on 433MHz (and others) and decodes transmissions received from known (433) devices
universalRadioHacker - General purpose signal processing/decoder