OBD-II offers a standard way to access many types of data, including:

• Real-time parameters: RPM, speed, pedal position, spark advance, airflow rate, coolant temperature, etc.
• Status of “Check Engine” light
• Emission readiness status
• Freeze frame: a “snapshot” of parameters at the time a trouble event has occurred.
• Diagnostic trouble codes (DTCs).
• Oxygen sensor test results
• Vehicle Identification Number (VIN)
• Number of ignition cycles
• Number of miles driven with MIL on

The SAE J1979 standard is the ultimate source for this information. Some of it is available for free from Wikipedia.