HackRF One

The HackRF One is an open-source SDR hardware platform that can transmit and receive signals between 1 MHz to 6 GHz. It can be used for a wide variety of activities, including replay attacks against car locks.

Michael Ossman, the creator of the HackRF One and the Ubertooth One, has a video series where he goes in depth on SDR and the HackRF One. You can watch his videos here.