Features:
ELM327 USB is the latest PC-based scan tool. It supports all OBD-II protocols and is dispatched with a number of compatible programs. The processor is an ELM327 USB. The output protocol (connection to laptop) is USB.
Processor: ELM327 USB

OBD-II Protocols:
 ISO15765-4 (CAN)
 ISO14230-4 (KWP2000)
 ISO9141-2
 J1850 VPW
 J1850 PWM

Output protocol: RS232
Baud rate: 9600 or 38400
Indicator LEDs: OBD Tx/Rx, RS232 Tx/Rx, Power
Operating voltage: 12V, internal protection from short circuits/overvoltages
Nominal idle current: 45 mA

Description:
OBD-II Software for ELM327 USB is a free program that allows you to use your PC and a hardware interface to get the information from your car's computer.

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.

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

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