links:

1 Short discription of Class 2 network
2 Multiplex Engineering, serial to J1850. ~$85 interface. No software.
3 Card Labs, some J1850 related specs available here.
4 Adv. Vehicle Tech. listing of J1850 devices. Chips listed here. Complete products are very expensive $800-1500
5 ScanTool.net $85 interface
6 ScanTool.net Free software & source code.
7 more links to specs and data sheets.

Serious prices, but snoops everything.
8 Vehicle Spy software. $1995
9 NeoVI vehicle interface for above. $1995

10 Freediag on sourceforge.