Old School Runescape
Specifically Old School, as (Modern) Runescape is a dumpster fire of MTX. Old School's only cost is a membership subscription that grants access to easily 10x as much content and game mechanics, but I was still able to get over a year out of finishing all the free to play questlines (and the stat grinding needed to unlock some of them).
There are a few different projects that allow you to self host specific older versions (I think 2009scape and 2006scape), but "Old School Runescape" is an official fork of the game (from before they added MTX and overhauled most of the mechanics for the worse) that is supported by the parent company and getting new non-MTX content updates still (but mostly to member only stuff).
It was pretty much the poster child for browser based MMOs back in the 2000s. I believe it is still playable in browser, but most people play through a client now.
Last I checked there is the official client available through Steam, an official client for Android and iPhone, a purely graphics overhaul focused client called HDOS, and a developer approved client called Runelite that has an absolute ton of plugins (qol, graphics enhancement, various integrations with twitch/discord/etc) which have been verified by the devs as not cheating, since there's a big PvP scene.