Multimeters, Function Generators, Oscilloscopes, Logic Analyzers, Spectrum Analyzers, Software Defined Radio's and Open Source/Community Designed Tools. If you used it while reverse engineering, list it here!
Software Defined Radio’s
RTL-SDR - Very low cost (~$30) receiver operating from about 52MHz to 2200MHz.
SDRPlay RSP1a - Medium cost (~$160) receiver operating from 100 Khz to 2000 MHZ. Device includes a lot of selectable filters (to cancel broadcast FM, WiFi interference, PC interference etc).
HackRF One - Capable of transmission or reception of radio signals from 1 MHz to 6 GHz. Open source hardware platform that can be used as a USB peripheral or programmed for stand-alone operation. Cost ~$340
Ettus Research USRP B200 - Universal Software Radio Peripheral with continuous frequency coverage from 70 MHz –6 GHz. Wider bandwidth and higher resolution ADC/DAC than the HackRF; cost with enclosure ~$875.