They also sell them on ebay I think. They are super cheap and I've heard some have had issues, but I've ordered several and all have worked great. I use the torque app with my android device. I've used the app on the T4R and the Audi I had before it and it is very powerful.
They have a free version that can do quite a bit. I opted for the pro version for some of the advanced features. Beyond reading DTCs and clearing them, you can access a lot of information coming into the ECU from various sensors. I used it on my Audi to confirm that I had a thermostat stuck open. I was able to record the sensor data and plot it in excel later to analyze it.