I hope others enjoy reading information such as this. I have been tossing the idea around of buying a HackRF for awhile now and even went so far as to order a HackRF Blue, but the Paypal nightmare made it to difficult to actually buy one.
Personally I wouldn't mind seeing a forum/area for SDR information here but am not sure who to ask about getting one set up. :-) With BeagleBone, Arduino and RPi enthusiasts I am sure people would be interested in reading and inputting their own experiences on all 3.
But since this is specific to RPi I will put it here.
A few days ago we posted about RpiTx, a piece of software that allows you to turn your Raspberry Pi into a multi purpose transmitter by modulating the output on one of the GPIO pins.
Now over on YouTube user HA7ILM has uploaded a video showing his related software qtcsdr. Qtcsdr runs on the Raspberry Pi and interfaces with an RTL-SDR dongle and RpiTx to create a simple transceiver radio. In the video HA7ILM shows the software in action by using a microphone and RTL-SDR plugged into the Raspberry Pi, and showing the microphone transmitting via RpiTx and being received via the RTL-SDR.
Qtcsdr can be downloaded from https://github.com/ha7ilm/qtcsdr.
As always with this type of thing only transmit if you are licensed and take care with the transmitted distance and filter the antenna output when transmitting over a distance that is further than your room. Also regarding this, on the qtcsdr GitHub page the author mentions that a Raspberry Pi shield called the QRPi filter + amplifier is currently in development (white paper).
QTCSDR Control GUI