I have been trying to gather momentum to build one for a PDA, but having trouble getting the protocols for the existing hardware (PT, LS1M) and open sourcing it.

But, for the time being, this is probably the cheapest and maybe the fastest one out there, and updates are free.

OBD-2 Vehicle Explorer Scan Tool Browser

The interface is kind of hard to follow, and only works with laptops, but the guy knows his stuff apparently.

Shawn