There's a few. A big reason why Speeduino and RusEFI exist as open source projects is that Megasquirt, whilst they make the source available for viewing, is very much NOT under an open license. You can't use it on anything but their hardware, can't modify it, can't copy it etc. It's also quite hardware specific with big chunks of assembler in it.

Speeduino started as a way of getting a DIY ECU that was far cheaper than what Megasquirt offerred and it's currently about 1/3 the price. It's not on the same level for features at the moment, but I do new firmware releases every 2 months or so with more functionality etc.