Gqrx
Gqrx is a software defined radio receiver powered by the GNU Radio SDR framework and the Qt graphical toolkit. Gqrx supports many of the SDR hardware available, including Funcube Dongles, rtl-sdr, HackRF and USRP devices.
gr-scan
gr-scan is a program written in C++, and built upon GNU Radio, rtl-sdr, and the OsmoSDR Source Block. It is intended to scan a range of frequencies and print a list of discovered signals. It should work with any device that works with that block, including Realtek RTL2832U devices.
kalibrate-rtl
Kalibrate, or kal, can scan for GSM base stations in a given frequency band and can use those GSM base stations to calculate the local oscillator frequency offset.
KillerBee
KillerBee is a Python based framework and tool set for exploring and exploiting the security of ZigBee and IEEE 802.15.4 networks. Using KillerBee tools and a compatible IEEE 802.15.4 radio interface, you can eavesdrop on ZigBee networks, replay traffic, attack cryptosystems and much more.
Kismet
Kismet is an 802.11 layer-2 wireless network detector, sniffer, and intrusion detection system. It will work with any wireless card that supports raw monitoring (rfmon) mode, and can sniff 802.11a/b/g/n traffic. It can use other programs to play audio alarms for network events, read out network summaries, or provide GPS coordinates.
Multimon-NG
MultimonNG a fork of multimon. It decodes a large number of digital transmission modes.