RTLSDR: the little radio which does so much for so many people around the world. It is inexpensive, light on power consumption, and tunes quite a large swath of radio spectrum. With proper software on its host computer, it can bring you dozens of types of signals: broadcasts, aircraft communications, amateur radio, military or emergency services, and even weather maps.
RTLSDR hardware is manufactured in two basic flavors: those with the E5000 tuner, with a zero-IF, quadrature sampling architecture versus a modernised circuit containing variants of the R820 tuner. The latter used a heterodyning method to downconvert the signal to a low IF, which is sampled in quadrature.
It is a sensitive radio, which works well on beam, loop, or dipole antennas, outputting a stream of 8 bit I/Q samples for demodulation or decoding on the client computer.