One of my absolute favorites is a quirky strategy game named Lord Monarch. I found it on the Nihon Falcom website way back in the mid 90s when I was first starting to use the internet: Lord Monarch

It's super tiny in modern terms, but I remember waiting a good 20-30 minutes for it to download from a 33.6Kbps modem.

Another huge one for me was a game I actually got a full version of from a Gravis gamepad promo CD named Seek and Destroy. It's a top down shooter where you can be either a helicopter or a tank: Seek and Destroy

The last one that comes to mind is a giant robot fighting game that also came on that same Gravis CD named One Must Fall: One Must Fall 2097

UnReal World is a free, survival-focused roguelike rpg that came out in 1992 and is still actively being developed and improved. It’s got a lot of depth and a very cool fantasy setting heavily inspired by Finnish folklore (the developer is Finnish).

I highly recommend it if you’ve got the patience for an in-depth roguelike rpg.

Irisu Syndrome! is a favorite of mine. It's only from 2008, so I personally don't consider it retro. It's tricky to learn at first since it doesn't tell you anything about how to play and I don't recall there being a manual. It's also one of those games that you can't really say too much about without spoiling things.

Wolfenstein Enemy Territory, or the modernized remake, ET Legacy

For the longest time I thought Minecraft was free because I only ever pirated it. Lol

PlanetSide 2 is the free-to-play I keep going back to

Haven't played recently, but this one came to mind'

https://en.wikipedia.org/wiki/Scorched_Earth_%28video_game%29

Squarez Deluxe

Makes Tetris seem like a children's toy

If we're sticking strictly to free, I'm not sure Doom mods work if you only have the free version installed. Either way, people basically never stopped making wads since the game came out, and I try various ones out from time to time.

There are tons of them, so if you're interested you can see what won the annual Cacowards throughout the years and start there.

https://doomwiki.org/wiki/Cacowards

Super Crate Box. An incredibly addictive, simple 2D shooting game! It's free on Steam!

"Einstein puzzle" by Flowix games, copyright 2004-2005

I play it from time to time, to see if I can still complete it in under 4 minutes and my brain isn't slowing down too much with age.

Technically, Dwarf fortress is the oldest game I still play, but I only got fully into it thanks to the non-free Steam version.

Tyrian is still fantastic 30 years later!

Beneath A Steel Sky (though I kinda prefer the grungier aesthetic of the Amiga floppy version)

Frog Fractions

I don't play it regularly, but it's fun to tell people about it.

I don't know if it technically counts as freeware, but Space Cadet Pinball. Back on XP it was my go-to for something quick to play on the offline computer, and I just brought it with me to every new machine. Also it's now been decompiled and will run on pretty much anything!

Beyond All Reason!

Something about Castle of the Winds really seemed unique and even today it's distinct from most roguelikes. I liked the fact that items could be enchanted or cursed. Simplistic graphics, but fun! I also liked the 3D pinball game (demo) that came with an old version of Windows...

OpenTTD is open source, so not sure that fits.

Dwarf Fortress, since 2008

ASCII no questions, I'll tell you no lies

https://supertuxkart.net/

I've been playing it off and on since maybe 2016-18 (somewhere within that range), so a pretty long time for me, but Pokemon Rejuvenation.

If you don't mind swearing, death, and generally kinda edgy stuff, it's a fun enough game with a lot of content. 15 out of 18 badges (one per type) are currently available, with a very long story to go with it.

Just started over on my Steam Deck, making this my umpteenth time starting over, because I felt like it and wanted to do a grass only run, similar to my fire only run. Also, unlike just about every other RPGMaker XP fangame, the devs have a Linux version available, which has an appimage instead of the .exe file to run the game and have made a bash script to update the game to replace the .exe file for that. Wine not required to play at all!

Every time I mess with a Mac emulator I play some Snood. I think it was only free on that platform though.

bubble tanks

Desert strike? Not sure if it was free tho. Also road rash

Not freeware, but currently $2.39 on GOG:
Master of Magic

Its like Civilization with wizards and dragons.

They're shareware but I think Pocket Tanks and Super DX Ball have been on every computer of mine since 2002

The Chzo Mythos games. Point and click adventure series made in AGS.

I first played 5 Days A Stranger when it was out, and it was my favourite adventure game.

Then 7 Days A Skeptic, which was basically more of the same and I thought it was very cool.

After that is Notes, which is a text parser instead of point and click. Still a lot of fun, but I always use the guide to help myself in parts.

Last one was 6 Days A Sacrifice, which I don’t remember very well, I haven’t played it as much as the other games.

Also the same guy made a game called 1213, which I loved. It played more like Flashback, cinematic platformer style.

I play Shareware