Picked up one of these
DealExtreme: $39.75 ELM327 USB Vehicle OBD-2 Scanner Tool
$39.75, from Deal Extreme (free shipping) seen elsewhere for quite a bit over $100.
Works with Scantool.net in XP Pro but can't get it going with Vista. Had to download the virtual COM port drivers from ftdichip.com 'cause the CD-R included only had drivers for Win 98 and 2000.
I've tried sending commands to it and used the test programs from scantool but all that happens on Vista is the USB TX light blinks.
I had to borrow a laptop with XP to see if it worked, can't do that all the time! I need to get the thing to work with Vista Ultimate 32bit.