Open Source OBD software

JDash http://jdash.ludicrous-speed.com/wiki/index.php?title=Main_Page
Opendiag simple OBD-II to RS232 cable and information about Volkswagen-specific extensions.
Freediag open source software that works with six different cables.
the classic "Jeff" interface OBD-II <-> RS232
scantool.net interface vendor offers source code for Windows that is also reported to work in Linux under Wine. How to use the software
Design News article on improving fuel efficiency includes an OBD-II interface circuit and a discussion of how fuel efficiency can be calculated from airflow and vehicle speed.
obddig.net offers a schematic, software, and a kit for a PIC-based computer interface.
Serial interface OBD II or KL to RS232 with cheap IC MC33290 (schematic and PCB; German, translated with babelfish)
USB Port interface OBD II or KKL to USB with cheap ISO IC L9637 (schematic; German, translated with babelfish)
RomRaider ROM editor/ECU logger for Subarus