What can be done with OBDII?
 

I can not currently efford an OpenPort or AP.
The only thing I can get is an OBDII interface (either USB or Bluetooth). It will be a chinese version of ELM327.
What can be done with it? Can I do some dataloging to make sure my car runs fine (standard setup of a STi with a ProDrive kit)? Can it be used to a PSI3 type data monitor? (realtime)?

I presume you've got a newage car given it's a Prodrive STi? Don't bother with anything ELM-based as these only work in OBD2 mode which is very limited for logging. You need something capable of handling the Subaru Select Monitor protocol, which means you can pay for an OpenPort or AccessPort, or you can use a "basic" ISO9141-compatible OBDII interface, along with DeltaDash, EvoScan, ECUExplorer or similar software.

I have an '03 STi.
I dont understand something. This EBAY auction of an chinese BT OBD adapter claims to support iso 9141 (BLUETOOTH Scanner EOBD OBD2 OBD 2 AUTO CAN-BUS WIRELESS:eBay Motors (item 320423933668 end time Sep-16-09 12:30:11 PDT))
What does it mean?
What cables can suit my needs? Any schematics availble so that I can build one my self?

Mitsubishi OBDII DataLogger - EvoScan
The $57+£10p&p one should be lots better than any OBD cable as it should work with SSM - I hope so anyway! I ordered one at the weekend for my '99 UK Turbo2000 and eagerly awaiting it arrival. I'm a bit disappointed I can't tweak (ruin) my engine myself - but at least with this I can have a look at what is going on :)
It should also work as an OBD2 cable for Mrs Bigun's car too.

Sheeez, this is one of those subjects that's far more complex than it should be to understand, primarily because the motor industry turned something that should have been quite simple into a mess.

Suffice to say that OBD2 is a "generic" communications format, while Subaru Select Monitor protocol is, as it sounds, specific to Subaru. It works faster than OBD2 and gives access to more detailed information from the ECU.

Start by reading this, and note the difference between "intelligent" and "dumb" interfaces. The intelligent interfaces do some of the job of creating the diagnostic commands themselves, while the dumb ones simply act as a conduit, meaning that the software connected to them does all the work of creating the commands and decoding the response from the ECU.

This sounds good in theory, but it's a problem if you want to use SSM as the intelligent interfaces only speak generic OBD2. If you want to use SSM to analyse your car (and you do, you need a dumb ISO9141 interface.

The USB ISO/VAG-COM interface on Andy's page above should work fine with any of the Subaru-specific diagnostic/logging suites you're likely to use. ECU Explorer, if you can still find it, is freeware and allows logging.

So.... basicly I have two options:
Pay ~180USD for OpenPort2
Pay 20USD for VagCom cable?

Sound strange.... What does OP do that VagCom doesnt?

The latest OpenPort is CAN and Mitsu compatible I believe. It also has the extra bits needed to automatically supply the VPPH voltage necessary for ECU reflashing - although you can manually replicate this on a dumb ISO interface with a couple of wires and a switch.