Listr.lol, highliter, and Amethyst have lists for users

"Categorized People list: (kind: 30000) This is a list of users with a title. Hence, you can store any type of people list you'd like."

Useful for organizing users into their own circles. Especially with a lot of follows. 

 Nip-51

https://github.com/nostr-protocol/nips/blob/master/51.md