Facebook like friends system. Where your friend list isn't public, and content is encrypted so only they can see it.

Instagram, YouTube, SoundCloud and anchor.fm alternatives using bittorrent + nostr.

Reddit like moderated public communities.

Patreon, OnlyFans alternatives.

Pay to DM system.

WordPress like blogging system with custom design and shops and whatnot, but based on nostr keys so you can move your blog around without loosing your readers/customers.

Twitch, Clubhouse alternatives (Jam + nostr?)

Tinder alternatives and other such apps for meeting people IRL without doxing your location to everyone

Omegle alternatives (nostr + some live streaming tool) and reputation for spam prevention

Discord on nostr

GitHub on nostr