My preferred .NET libraries/tools

FluentResults - For Result/Option Pattern
EntityFramework - ORM
FluentAssertions - Fluent API for Asserts
Scrutor - Extend DI scanning
Dapper - micro ORM
XUnit - Test framework
FakeItEasy - Mock framework
CsvHelper - helps with csv's
Polly - retry/short circuit mechanisms
Bogus - gibberish data for testing
Hangfire - background jobs with persistance
NServiceBus - distributed applications 
Serilog - structured log
Refit - automatic REST library

dotnet-outdated-tool - CLI to inform which packages are outdated.
dotnet-ef - CLI for Entity Framework for migrations, etc.