Warzone-2100
It’s been a few years, but Armagetron Advanced was the best. Felt like you were riding the lightbikes from Tron.
OpenTTD
Lost Vikings
I often revisit Soldat
Nikujin by ikiki, and other games by the same developer. Not sure where to find them now, but they were some of the greatest freeware games I have played.
Clonk! It’s a 2D mining/strategy/base-building hybrid that was apparently more popular in Germany than other places. I actually paid to play it way back in '05, but it has since become freeware. It’s got a particle sim (similar to Noita, but not as extensive) that plays into the scenarios well.
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.
Wolfenstein Enemy Territory, or the modernized remake, ET Legacy
Haven’t played recently, but this one came to mind’
I missed the original, but Scorched 3D was a big hit at my high school.
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.
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.
Super Crate Box. An incredibly addictive, simple 2D shooting game! It’s free on Steam!
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!
