DOOM and Quake are the most iconic classics for various reasons in my opinion. One of the best reasons is that their engine source code was made open source. Not only did a large company like steam use the open source engines, but the game communities also picked up the public code and made their own source ports.
It is important to remember that a game has two core components: the game engine, and the game data or assets.
In the case of DOOM or Quake, the game engine is the only thing that was made open source. The game content is still required to be purchased. What this means is that you only have to legally purchase the game once (can get from gog.com) to obtain the game data.
A game like Unvanquished however, is entirely free and open source (both engine and game data).
I compiled a list of games that I found enjoyable, and was able to compile from source rather than relying on a pre-compiled binary.
#free #open-source #games