

Star Wars Jedi Knight I and II: Jedi Academy and Jedi Outcast (Not stable) - OpenJK https://github.com/JACoders/OpenJK
Despite the official name being “Jedi Knight: Jedi Academy” it is not Jedi Knight I. It is actually a sequel to Jedi Knight II: Jedi Outcast. It came out only a year after JO and was announced in April. At first I thought it was an April Fool’s joke because the screenshots looked like something that could easily be made with the mod tools at the time.
It wasn’t called Jedi Knight III because you play a different character.
Anyways, the real Jedi Knight I was “Jedi Knight: Dark Forces II”. An open source implementation is available at https://github.com/shinyquagsire23/OpenJKDF2
And to round it off, the open source implementation of Dark Forces is available at https://theforceengine.github.io/. I think it even supports the remaster.
Oh, also ScummVM should always be included in such a list. It supports a ton of classic adventure games from LucasArts, Sierra and others. And by now even Might and Magic 4+5: World of Xeen.