What are some less known or underused open source fonts?

I love this hyperlegible one from the Braille Institute: https://www.brailleinstitute.org/freefont/

I'm going to see if I can get this on my Kobo. Thank you for the link!
Ooh there's a monospace version too
- this is the way
I use this, it's good.
I love this font I was going to say this too.
It doesn't take my duck.com alias
Came looking for this comment! It’s my favorite font of all time. So clean and legible.

Shout out to 0xProto! Texture healing in a monospaced font is super cool!

Interesting! I hadn't heard of this before, I'll have to check it out.
and actually useful ligatures to boot! neat.
Rad!

There are too many to mention since there's so much out there.

You can go to a font website like dafont and perform a search for a font or browse different themes/styles and specify in the results that you want Public domain / GPL / OFL fonts.

Almost all of my fonts are released under an attribution license, so they're free for commercial use and remixing.

https://michaelwmoss.com/typefaces

Definitely a lot of fonts I'd very much like using whenever I need a new font next. Thanks!

Agave. My favorite monospace/programming font. What I like best about it is how the baseline isn't where the descenders begin. Makes it look unique in a nice way.

Merriweather is great for longer texts. I use it in all book reader apps.

Anonymous Pro is my coding font of choice.

I second the Anonymous Pro!

For those who love comic sans I might have bad news: you might be borderline dyslexic.

Switching to OpenDyslexic 3 helps

OpenDyslexic is less good for most dyslexics than you think. It's based on some ideas of what might be useful rather than specific evidence. I recomend Sylexiad instead. Particularly Sylexiad Sans rather than Serif, but it's all about finding what works best for you.

M+ 1m has been my daily monospaced font ever since I found it, which is about eight years now. I don't see it come up often when people talk about monospaced fonts.

I see lots of links here but no reasons why each is underused or anything.

Look, I'm a simple person, what I ask of a font is:

differentiate your symbols — I'm talking 1, l, I, and i with more than a single pixel difference at 11pt 90DPI, and the 0 should be clearly less round than O
be readable. No extra thinness, fancy swirls... look at any default font as an example (Arial or whatever), or most people's custom web font as a counter-example
proportionally spaced. I like \thinspace as thousand separator and I cannot lie
indefinitely usable (for example as part of a game) after a single purchase that I can do as a consumer. Or free/donationware of course

A very tall order I know. So far I've reviewed a bunch of fonts (I wasn't procrastinating why do you ask) and found PT Sans is the best option I've seen, so I'm using it, but I hate its Q. It's basically an O with a tilde below it. Any better options if all you want is clarity and normalcy?

Edit: near the bottom of the replies there's Hyperlegible. Somehow I had read over that. Seems to check the boxes! I'll be looking at this closer on my computer later.

This is a new one: https://github.com/sayyadirfanali/Myna/

I don't know it it's underused, but my go to for years gas been League Spartan from The League of Moveable Type

Ärzte Sans (edit: missed the open source part. I can't verify that)

Looks to be.
- Huh til theres an aur package, I just plunked it in my fonts directory

I'd say several fonts on Google Fonts (the only place I know lol) are pretty good. I don't see them mentioned anywhere and I'm not a font enthusiast per se, so I'd call them less known. Some of my favourite - Courier Pro, Doto, Sono, SUSE Mono, Josefin Slab, and Martian Mono.

And a couple others default on my Windows PC that I like: Bookman Old Style, Yu Gothic (Fun fact - this font was used by the hit anime Neon Genesis Evangelion for their english title cards) and wierdly enough, Segoe UI. I love that Windows 8 aesthetic it brought.

I really like Open Sans

Muli !

My favourite

Search at http://openfontlibrary.com/

Comic Neue

It's freeware not open source but I like Monofur. Might not be the most beautiful font but I find it very legible and the distinction between similar characters is quite good. It's available in the Debian/Ubuntu repositories too.

This doesn’t add anything to the post, so, sorry, but this is a really good question! Thanks for contributing!