There aren’t many. nostr:npub1jlrs53pkdfjnts29kveljul2sm0actt6n8dxrrzqcersttvcuv3qdjynqn recently opened a pull request to update NIP-52, so I guess it’s either Coracle or Flotilla that he’s working on that supports calendars.

I’m building Comingle, an events app which was my original motivation for the NIP, but that’s on only iOS at the moment. Eventually, it’ll support other platforms.