PC version is completely free and has no ads or microtransactions, mobile version has those.
Cho Ren Sha 68K
The Windows version was removed from the dev's website as part of the version 1.10 update. 1.10 is X68k only, but it's also free; you will need an emulator to run it on Windows and possibly poke around some settings.
Eden's Aegis
Noiz2sa
Parsec 47
The port on Blast Works is not free. PC version is.
rRootage
Do note that there are console and mobile ports of this game that cost money, but the PC original is free.