Mbin? NodeBB? Misskey? Friendica? Loops? Pixelfed? Mastodon? (not compatible with Lemmy but PieFed and especially Mbin can interact with it fairly and extremely well, respectively) Flarum? (one day, not yet afaik)

Edit: Pleroma, Funkwhale, BookWyrm, and so much more - the Threadiverse is just the barest beginning of the tip of this iceberg! :-)