The oldest one I regularly use is this one: https://web.archive.org/web/19961108115333/http://www.kela.fi/
It's the Finnish state organ for giving people their welfare benefits and operating the national health insurance system. So, if you get a child, you get an amount of money per month from Kela for having a child. (Because they want the population to grow).Or, if you end up unemployed for a longer time, it's Kela that will pay your unemployment benefits.Of if you break your leg, it's Kela that pays for your hospital care. I don't know how many forms I've already filled on their website :D
But yeah, the site's been around for a while now.
When starting here, I found it a nice way for finding interesting communities to go to different instances' pages with my web browser and browsing their Local. A lot of communities I would never have thought of!
So, go to sopuli.xyz and see what their Local has to offer, then go to piefed.social for the same. And suppo.fi and Solarpunk (is it slrpnk.net or what?) and what is there still? Blåhaj at least. You can see what instances interesting commentators are coming from and check out the rest of that instance's atmosphere by just writing that instance's address in your browser's URL bar.