I have 2. An eBay cheapie to link with the free version of Torque for getting readings, and a
Bluedriver, which is ~$100US, that can scan & clear codes for all the computers*, not just the ECU. It also supplies diagnostic info on common causes of codes that users have reported back to them.
The Bluedriver was very useful when I was chasing ABS error codes as they wouldn't show on the dash display. Ended up being just a failed sensor.
The eBay cheapie + Torque helped to figure out that it was the cats failing not the sensors by looking at the O2 sensor outputs on a graph. (Because the Bluedriver module won't talk to Torque.)
*Most cars have at least 3 these days, ECU, ABS, Transmission...