[MegaSquirt] is the original 8 bit Motorola implementation of an affordable electronic fuel injection (EFI) system by Bowling & Grippo.

Although any other EFI systems could have inspired MegaSquirtAVR, (they have similar sensors and similar algorithms) this design was the inspiration actually. For more flexible design the hardware (schematic and PCB) and firmware needed to be implemented from the ground up, but MegaTune (MegaTunix) compatibility is maintained in the communications protocol, so the fantastic MegaManual documentation and experience can be taken advantage of. Since then MegaSquirtAVR has grown and expanded. The board is called GenBoard and the groups that sponsors it's development is this site.

The megasquirt had several forks to provide additional features, most notably:

Unfortunately the MegaTune line is not unified, there are about 10 different megatune implementations to support the extra features of firmware forks. MS-AVR have a GenBoard/MenuSystem to configure the extra features, but can use MegaTune (any versions) to set the basic features (most useful for the VEtable and other 2-dimension tables).

[Colin's website tries to make clear about all existing megasquirt variants] - GenBoard being the most advanced, but much experience from others can be used.

The megasquirt core developers realized that they need to follow similar route as the MS-AVR team to get rid of the inherent limitations of the assembly codebase, so they started the reimplementation called Ultra-Megasquirt (UMS), which will provide similar features as MS-AVR, but besides expectation nothing is available yet.