This is a new version of the stimulator originally designed by James Murray (jsmcortina on the forums) that expands on the original stim's features. The main improvement comes with the addition of a microcontroller which allows the simulation of up to 30 different toothed wheels, in addition to the standard distributor signal of the standard stimulator. Instead of having the Megasquirt set to "Stim for Wheel" to test your RPM input on the Stim, you can tell the JimStim to provide a signal like the one from a Neon, Miata, Subaru, most Fords, and many more. It can also drive ignition input mods with pull-ups. At the moment, there are 23 programmed wheel patterns but the firmware is upgradeable using a specific USB cable made by FTDI.
The improvements don't stop there. James added a set of user-configurable jumpers that let you wire it to test IAC motor outputs, spare inputs, and spare outputs. You can wire these up with the selection jumper wires. There is also a bank of screw terminals that let you connect things other than a standard Megasquirt to the JimStim. So if you're looking for a Stimulator to test out your Microsquirt, eMS-Pro, or other Megasquirt devices that don't have the standard DB37 connector, you can wire one of these up to a JimStim to test it out.