Steam (Windows, Mac, Linux):
Link

Steam:
Link
More info, a non-Steam Windows port, and the many fan games:
Link

There is a mobile version, but it costs money.

Windows, Mac, Linux (and other ports):
Link

Steam (Windows, Mac):
Link

Steam Deck:
Link

Steam (Windows):
Link

Windows, Mac:
Link

Windows:
Archive.org Link

I haven't tested it, but there's apparently a community-made frontend to help with installing the game and various mods.

Steam (Windows):
Link

Basically just Vampire Survivors, but with Hololive Vtubers and free.

Windows:
Link

Okay, it's not exactly "great," but it's interesting and worth checking out.

Steam (Windows, Mac):
Link

itch.io (Browser):
Link

Was later included in Celeste (2018)

itch.io (Windows, Linux):
Link

It's recommended that you play Celeste (2018) first.

Steam (Windows):
Link

Steam (Windows, SteamOS):
Link

Steam (Windows, Mac, SteamOS):
Link

An accurate depiction of pineapple on pizza. Lasts only 10 minutes. It's better the less you know going into it.

Windows, Mac, Android, Linux:
Link
Check out the awesome addons available too:
Link

Windows (and soundtrack download):
Link

It is strongly recommended that you play Undertale first.

Steam (Windows):
Link

It does offer paid-DLC, but it's just an artbook and wallpapers.

Windows:
Link
Browser:
Link

Windows, Mac:
Link

Steam (Windows, Mac):
Link
itch.io (Windows, Mac):
Link
Nintendo Switch eshop:
Link
PlayStation 4 Store:
Link

Technically a demo, but a substantial one. Each chapter lasts a good couple hours at least.

It's strongly recommended that you play Undertale first.

Steam (Windows, Mac, SteamOS):
Link

Windows, Mac, Linux:
Link
Addons:
Link

Steam (Windows):
Link

Windows (64 bit & 32 bit), Mac, Android:
Link
More info and soundtrack download:
Link

itch.io (Windows, Mac):
Link

Steam (Windows, Mac, SteamOS):
Link

A sandbox business sim.

Browser (requires Flash), Windows (1.0 download):
Link
Newgrounds version (requires Newgrounds account):
Link

Steam (Windows):
Link

Steam (Windows)
Link

CW: "This game contains depictions of violence, gore, child harm, child endangerment, child death, and suicide. This is a horror game about disturbing content."

itch.io (Browser, Windows, Mac, Linux):
Link

Steam (Windows, Mac):
Link

Browser (Flash):
Archive.org Link

Windows download (and soundtrack):
Link

Steam (Windows):
Link

It does have DLC you can pay for, but it's basically just to support the developer. You get next to nothing for buying it.

itch.io (Windows, Mac?):
Link

Browser:
Link

I'd also recommend reading the mental health tips linked at the end of the game.

Steam (Windows):
Link

Steam (Windows):
Link

A metroidvania bullet hell.

Windows, Mac, Linux:
Link

A more maximalist game than SRB2 Kart. I can't say which of the two you'll prefer. Your mileage may vary.
"Isn't the tutorial super long though?" They patched it.

Windows:
Link

Steam (Windows, Mac, SteamOS):
Link

Windows:
Link

This is the 2014 version. The 2016 version and the World Machine Edition cost money, but you get to continue if you close the game.

Windows:
Link

Steam (Windows):
Link

CW: "Psychopomp contains grotesque monsters and disturbing scenes. Sounds and visuals are used to produce a disturbing psychological horror environment. "

Steam (Windows, Mac, SteamOS):
Link

Great for learning Javascript.

Steam (Windows, Mac, SteamOS):
Link

A pixel FPS by Vlambeer.

Steam (Windows):
Link

Steam (Windows):
Link

A dark-fantasy roguelike.

Browser (via BBC website):
Link

Steam (Windows, Mac, SteamOS):
Link

An Action game with physics-based combat.

Steam (Windows):
Link

Browser:
Link

Steam (Windows):
Link

Steam (Windows):
Link

An early access rhythm game / visual novel.

Steam (Windows):
Link

Steam (Windows):
Link

Steam (Windows, Mac, SteamOS):
Link

Steam (Windows):
Link

A visual novel RPG.

Steam (Windows):
Link

Steam (Windows, Mac, SteamOS):
Link

Steam (Windows):
Link

Windows:
Archive.org Link
MyAbandonware Link

Steam (Windows):
Link

Steam (Windows, Mac):
Link

Steam (Windows, Mac, SteamOS):
Link

An Adventure game "inspired by Umberto Eco's novel and Jean-Jaques Annaud's film 'The Name of the Rose', and based on the game 'La abadía del crimen' by Paco Menéndez and Juan Delcán."

Steam (Windows):
Link

Steam (Windows, Mac):
Link

Steam (Windows):
Link

Steam (Windows):
Link

An eccentric cosmic-horror RPG.

Windows:
Link

Steam (Windows):
Link

Steam (Windows):
[Link](https://store.steampowered.com/app/1896750/Cursed/}

An Action-adventure horror game.

Steam (Windows):
Link

Steam (Windows):
Link

Steam (Windows):
Link

Steam (Windows):
Link

A monochromatic roguelike RPG.