flarum, Misskey, Friendica, Mastodon, the list goes on - I don't know so much about non-Threadiverse ones but see e.g. https://fediverse.observer/ for more details.

Note that Kbin is (semi-)officially dead - there is only a single instance (in Poland, https://bin.pol.social/) that still uses it, and the last commit of its software was on Dec 20, 2023. Everyone else switched to its fork Mbin that is actively maintained. This community really should update its sidebar text, to drop Kbin and add PieFed and Mbin. Kbin was a great idea at some point - I never would have come over to "Lemmy" with its known reputation as the place where many of the trolls went who were banned from Reddit (it's... sigh... actually true), so Kbin was what convinced me to leave Reddit - and now Mbin carries its legacy onwards, but Kbin itself is extinct (I am not trying to be hyperbolic here, it just literally is).

There was also Sublinks but... its development got stalled by the main dev having a baby (its last commits were all in 2024 iirc).

PieFed is the hot topic though - all praise the PieFed. I admit to some bias in this regard:-P, however it is also backed up by the stats: not only do we get enormous updates practically weekly (often at least a minor one multiple times a week), but by comparison Mbin has about 700 MAUs (Monthly Active Users), compared to PieFed's ~1500 (more than twice that). Which should increase rapidly now that app support is in the process of being firmed up, especially experimental support in Voyager the #1 Threadiverse app. Somehow PieFed has surpassed Mbin and even Lemmy in several featured areas - and astonishingly even that of Reddit (whose development for many years now has focused on increasing profits, not necessarily offering things that their user base actually wanted) - mind you it is still being polished, and some (very few) areas still need heavy work (such as the search functionality).