On newer cars at the diagnostic port (under driverside dash) there are only 3 standards: Ford, GM and iso (foriegn and maybe chrysler). There are ELM chips available for all 3 protocols. From any of these 3 protocols you should be able to intercept a lot of information from the car's onboard computers. The other protocols are proprietary and added in addition to the 3 standard protocols. On older cars I don't think there was a set standard (other than OBD-1).

(I mean foreign "to me" as imported into the US)