PCM Hammer supports (a subset of) 1999-2007 GM vehicles... but what if you want to read and write the ECU firmware of other vehicles?

Subaru

2002 to ~2015

Tactrix makes OpenPort OBD2 interfaces and EcuFlash software.

There's a community of users and hackers at RomRaider.com

Mitsubishi

Unknown years

Tactrix makes OpenPort OBD2 interfaces and EcuFlash software.

https://www.evoscan.com/

Saab

See T5Suite, T7Suite, etc at https://txsuite.org/

BMW

M52TU and M54 engines (roughly the E46 era, I think?)

There are a handful of tools listed at ms4x.net.

That community also seems to be mostly at RomRaider.com

Mazda

Miata, 2006-2015 (NC era)

See https://github.com/speepsio/romdrop