Some interesting Nostr sites I've come across and bookmarked. I've excluded clients and relay software:

http://nostr.build/ - Image upload service that offers optimizations and privacy on images.

https://rsslay.nostr.net/ - Turn any rss feed into a nostr profile that will post updates.

https://nostr-fzf.netlify.app/? - Search for users or chat channels across relays

https://nostr.io/stats - Nostr relay network stats

https://www.nostr.guru/ - Get information off nostr public keys (users) or events. Great for checking for NIP-05 verification.

https://nostr.directory/ - Find people on Twitter who are also on Nostr and get setup yourself.

http://nostr.watch/ - Find and get information on relays globally.

Please add on if there are others you have found useful.