Manufacturer Specifications:

Features
ELM327 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. The output protocol (connection to laptop) is RS232.
Processor: ELM327

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 is a free program that allows you to use your PC and a hardware interface to get the information from your car 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 hard drive, and you 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.

Features:
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