Web app, specifically, or any apps?

I'm also familiar with these three (web) apps:

• Alexandrite: https://alexandrite.app/
• Mlmym (Old Reddit Lookalike): Not sure if there's an "open" instance of it, but several instances offer it. See https://old.lemmy.world/ for an example (not sure if SJW offers Mlmym or not).
• Voyager: https://vger.app/

Might also also check out https://lemmyapps.com/ which has a good list that you can filter by platform/feature.