CAN bus simulation (automotive purpose) - tried Arduino and SparkFun
The Arduino CAN shield is pretty cool if you want to build an embedded computer to play with CAN.  However, if I understand correctly, you are building a one-off tool for your personal use (or will merely use an off-the-shelf tool if one exists for the right price).  For that reason, I urge you to consider writing PC software rather than embedded software to interface with existing, low-cost CAN hardware interfaces.

Charlie Miller and Chris Valasek used this so called ECOM tool and wrote software which interfaces with it.  The ECOM tool appears to have an API and example driver.  Their car hacking white paper and software will give a pretty good starting point for interfacing a PC to a car.  Softing also makes PC CAN interface hardware, but I am having trouble accessing their website right now, so I won't link it.

There are also several full-fledged, commercial software tools for analyzing and simulating CAN on a PC as well.  Vector's CANoe is a well known (and expensive) tool for that purpose.