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

Steam (Windows):
Link

Windows:
Link
Browser:
Link

Steam (Windows, Mac):
Link

Windows:
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:
Archive.org Link
MyAbandonware Link

Browser (via BBC website):
Link

itch.io (Windows, Mac):
Link

itch.io (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.

Windows (and soundtrack download):
Link

It is strongly recommended that you play Undertale first.

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

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

Windows, Mac, Linux:
Link
Addons:
Link

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 download (and soundtrack):
Link

Steam (Windows, Mac):
Link

Windows:
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.

Windows:
Link

Steam Deck:
Link

itch.io (Browser):
Link

Was later included in Celeste (2018)

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

Windows:
Link

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

Steam (Windows, Mac, Linux):
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."

Steam (Windows):
Link

Steam (Windows):
Link

Steam (Windows, Mac):
Link