PieFed communicates with Lemmy. Same content, different platform. That’s one awesome thing about federation.
There is also mbin (fork of kbin), and Sublinks, which is API compatible with Lemmy so should be able to use Lemmy apps with it (from memory, this is what Beehaw are hoping to move to).
Pretty sure the user experience folk are screaming for a path to be built there but are getting ignored.