ELM 327 USB Tools

Free software:
Scantool.net 
  GM Mode 22 Scan Tool by Terry 
  OBD Gauge for PalmOS and Pocket PC by Dana Peters 
  OBD Logger by Jonathan Senkerik 
  OBD-II ScanMaster by Wladimir Gurskij (ScanMaster 3.52 - local copy) 
  obd2crazy.com 
  OBD2 Scantool by Ivan Andrewjeski 
  OBDII for ELM322 by David Huffman 
  pyOBD by Donour Sizemore for MacOSX and Linux 
  RDDTC by Pete Calinski 
  Real Scan by Brent Harris 
  ScanTest for Pocket PC by Ivan Ganev aka a-ser 
  wOBD by WDT

The software supports two platforms - DOS and Windows.
 
The program lets you perform the following operations:
 Read diagnostic trouble codes, both generic and manufacturer-specific, and display their meaning (over 3000 generic code definitions in the database).
  Clear trouble codes and turn off the MIL ("Check Engine" light)
  Display current sensor data, including: 
  Engine RPM
  Calculated Load Value
  Coolant Temperature
  Fuel System Status
  Vehicle Speed
  Short Term Fuel Trim
  Long Term Fuel Trim
  Intake Manifold Pressure
  Timing Advance
  Intake Air Temperature
  Air Flow Rate
  Absolute Throttle Position
  Oxygen sensor voltages/associated short term fuel trims
  Fuel System status
  Fuel Pressure
  Many others...
 
Upcoming Features: 
  Data graphing and logging
  Freeze Frame data
  Continuous and Non-Continuous Oxygen Sensor test results

The program is very user-friendly, and easy to learn. It is also very easy to install: simply extract the files into a folder on your computer's hard drive, and you're ready to go. To uninstall the program, simply delete the entire contents of the folder where the program resides.