[tompazourek]

I'd add:

* Hangfire

* Quartz.NET

* NHibernate

* AutoMapper

* StackExchange.Redis

* the Castle project

* xUnit

* NUnit

* Serilog

* SimpleInjector

These are all very popular long maintained open source libraries I've seen used in many projects.

[ryouko]

Another to add

* MassTransit

* AutoFac

* MediatR

* Elsa

* BenchmarkDotNet