Anybody know of a cheap (free, if I can build it myself) way to get the data out of the OBD connector?
obddiagnostics.com sells a kit for $38. Consists of pre-programmed interface chip, crystal, two caps for crystal, and OBDII connector. You still need some common parts to complete the build, these are just the parts that are not commonly available. Order page is here.
Scantool and obddiagnostics are different interfaces which are incompatible. Scantool seems to be the most popular one, as it consists of only a few components and a cheap pre-programmed ELM32* processor (320, 322 or 323, depending on car interface type).
Most OBD2-software out there has been written for ELM-based interfaces. Looks like most DIY-interfaces are ELM-based, too.
CarPC status: HW all done, SW needs tweaked. Hardware: VIA MII-12K, 512MB, 60GB 2.5", CW-8123 DVD-CDRW, 7" Lilli ts, Opus 90W, BU-353 GPS, 802.11b PCI, USB bluetooth dongle, AverMedia AverTV Cardbus Plus, Morex Cubid 3677 Software: RR, MM/FD