Hi,
There are quite some classic PC games that I remember from my youth that have been released as freeware in the meanwhile. I thought this might be interesting for some here. I'll list my personal favorites, but the links contain longer lists. I'm not counting games that were released as freeware from the start. https://www.gog.com/games?price=free&sort=title&page=1
Several are available on GOG (good old games). They patch old games such that they will work on modern systems, but for MS-DOS games, they simply use DOSBox (DOS emulator). My favorites:
Shadow warrior (FPS with a ninja from the era of Duke Nukem 3D)
Tyrian (fun shmup, freeware version was renamed Tyrian 2000)
http://scummvm.org/games/
ScummVM is an interpreter for many classic point-and-click adventure games. It only needs the data files from the game and runs on modern systems. Some that have been released as freeware are:
Broken Sword 1 & 2
Flight of the Amazon queen
Beneath a steel sky
Lure of the temptress
Drascula: The vampire strikes back
https://cncnet.org
The classic 2D Command & Conquer real-time strategy games have been released as freeware (listed below). They are no longer available on the Westwood/EA website, but they can be found on third-party websites. The original versions work in DOSBox, but you'll have a better experience using the patched Windows versions from the website above. They work on modern Windows versions (including 10) and give you higher resolution, which is very handy to see more of the map.
Command & Conquer (original one, also known as Tiberian dawn)
Command & Conquer: Tiberian Sun
Command & Conquer: Red Alert
Command & Conquer: Red Alert 2
https://github.com/id-Software
id Software has released most of its major classic titles as open source, which has allowed volunteers to port it to other systems and operating systems.
Wolfenstein 3D
Spear of destiny
Return to Castle Wolfenstein
Wolfenstein: Enemy territory
Doom 1 and 3
Quake 1, 2 and 3
https://en.wikipedia.org/wiki/List_of_commercial_video_games_released_as_freeware
Wikipedia lists even more freeware games. Some good ones that are not on the sites above:
Alien Carnage / Halloween Harry (platform game)
Clyde's Revenge (platform game)
One Must Fall: 2097 (fighting game with robots)