Yes, Free and open source don't exactly mean the same thing. I'm not here to quibble. There are lots of lists of "the best" free and open source games. This is my much shorter list of free and open source games that I've played and liked. Specifically, this page tends to be about remakes rather than altogether new games.
These sites were remaking classic adventure games at a time when the world had forgotten them. Sadly most of them were hit with cease and desist warnings when the properties were bought and revived but at least a few of them were completed.
Note: Downloading random games from the internet stopped being safe quite a while ago and while I personally know and trust these games, I don't really know how you could feel safe downloading them now.
These guys made excellent remakes of
King's Quest 1, 2 and 3 as well as Quest for Glory II. Their remakes contain a lot of new material.
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AGI and SCI Adventure Games created by a Community of interested game developers. Some are good, some, not so good.
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This is a roundup of free adventure games and it's better than my roundup here. There's also a great Wikipedia page of Remakes.
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Here's some more places you can get free games.
List of Open Source Video Games
This is just a list of Open Source games but because it's on Wikipedia, it's quite up to date.
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