A big part of them are not really hidden gems, or they shouldn't for any dotnet dev that worked on a few projects before. They are on my mind on the "need to know"/"very important" list.

I am talking about FluentValidation, (maybe even) Ocelot, Polly (which isn't only about retries, and it's a very important tool if you work in the "distributed systems" world), MediatR (which if you are trying at least to do vertical slicing I see no reason why not to use it) and Dapper (which actually is a micro-orm not an actually orm, and even if EF7 almost got the same speed as this it, it still has it's "users")