GNU Radio is a free & open-source software development toolkit that provides signal processing blocks to implement software radios.
GQRX is an open source software defined radio receiver powered by the GNU Radio and the Qt graphical toolkit.
baudline is a time-frequency browser designed for scientific visualization of the spectral domain.
Universal Radio Hacker (URH) is a software for investigating unknown wireless protocols.
rtl_433 (despite the name) is a generic data receiver, mainly for the 433.92 MHz, 868 MHz (SRD), 315 MHz, 345 MHz, and 915 MHz ISM bands.