https://nostrplebs.com is another great one, run by nostr:npub18ams6ewn5aj2n3wt2qawzglx9mr4nzksxhvrdc4gzrecw7n5tvjqctp424

It’s different from the Twitter blue check. The check lets you know the user has published an address to use as an alias for their long public key. Ultimately it involves social trust in getting to know users first and learning that this alias represents them. Then the check mark has more meaning.