I remember there being three differant OBD standards, plus CAN-BUS. But it is 8 years since I worked in Automotive diagnostics, so there may be some new ones now too, plus all of the old manufacturer specific ones from before OBD was 'standardised'

The good thing about standards is there are so many to choose from!

EDIT: This was taken from the website of the company I used to work for "ECU Serial Diagnostics - ECU communications to industry standard protocols such as KW2000, KW81, KW82, Class II, or DS2 via CAN bus or ISO-9141 variants. Other application-specific protocols can be implemented on a per-project basis", so it looks like the industry is still far from having one universal standard.