The ELM327 allows you to turn your computer into an Auto Diagnostic System. This is all you need other than your car, the free software & your computer. Non-free software is also available. See the 'ELM buying guide' link below.

Features
  Read Codes: Display all Trouble Codes and Descriptions. Poll some codes more frequently than others. View minimum, maximum, & current values.
  Export Data for spreadsheet & printing.
  Clear Codes: Arrange, add, delete codes to your specifications.
  Monitor real time data in Metric & U.S. Standard.
  Record & Graph data.
  Performance Timer: 0 to 100 km/h
  Many advanced features!

Compatible Vehicles
  OBD2 protocols supported: J1850PWM, J1850VPW, ISO9141, ISO14230 (KWP2000), Plus CAN-BUS (J1939, ISO15765-4).

Specifications
  Includes 16 Pin Connector with Interface (Elm chip included) & Serial Port Cable.
  Elm Chip installed.
  Connects to your computer using the Serial Port.
  Compatible with J1850PWM, J1850VPW, ISO9141, ISO14230 (KWP2000), Plus CAN-BUS (J1939, ISO15765-4) Protocols.
  LED Status lights.
  Powered by the vehicles OBD2 Plug.
  Use it on a PalmOS, Pocket PC & Unix too! (on supporting software)

Software
   free software.
   Scantool User-Friendly.
   OBD Gauge for PalmOS & Pocket PC by Dana Peters.
   OBD Logger Windows & Unix by Jonathan Senkerik.
   OBD-II ScanMaster English, German & Russian by Wladimir Gurskij.
   OBD-II ScanMaster Pro
   obd2crazy Flexible Data Logging & Advanced User Tools.
   OBDII for ELM322 by David Huffman.
   RDDTC Simple Read and Reset software. by Pete Calinski.
   Real Scan Simple Read & Reset software good for older computers. by Brent Harris.
   ScanTest for Pocket PC by Ivan Ganev.
   RDDTC Simple Trouble Code Reader.
   EasyOBDII Low cost support available.
   GM Mode 22

  NON-free software
   Pocket Scanner Plus
   Digimoto
   PCMSCAN
   ProScan
   WGIobdII