HackRF is an upcoming open source DARPA funded ~$300 software defined radio that has both transmit and receive capabilities. It has a 100 MHz to 6 GHz range, 20 Msps sample rate, 8-bit resolution, and 500 beta units are about to be given away for free by greatscottgadgets. A good introduction to the HackRF can be found here, and on this wiki here.

Register here for a chance at receiving a free beta unit.