Mathstodon is cool, and it allows LaTeX formulas in your toots: https://mathstodon.xyz/about

And if you are into data science and especially data visualisation, also check out vis.social: https://vis.social/about

Both are also well run, IMHO.

https://mastodon.technology is chock full of geeks, so the local timeline is all people's hacks and bugs and side projects. It's one of the older instances, and it's maintained by this guy: https://ashfurrow.com/

For *BSD - https://bsd.network/

BTW, maybe exists something like to the "Awesome-Mastodon-Instances" for IT? BSD have instance, Ruby have instance, but how about Python, Linux, Computer Graphics, Game Development and so on. :-)

I picked https://mstdn.io/ because it's a medium-size instance hosted in Europe, the sysadmin seems very competent, and the site rules seemed reasonable (especially where it says "We only block/silence foreign accounts and instances for legal reasons.").

There are quite a few instances where a capricious admin gets to decide what content is acceptable for all their users. Even less transparent and less accountable than corporate censorship.

https://toot.io is a new free instance.

https://ruby.social is geared towards Rubyists but by no means is it restricted to only that topic.

sunbeam.city a solarpunk instance.

icosahedron.website and scholar.social are the ones I frequent, I like both a lot.

linuxrocks.online is a cool tech-focused instances.