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This is (no longer?) true, as others have mentioned. Just a few examples of great third-party packages that I use for many projects:

* MoreLinq

* Newtonsoft.JSON

* CommandLineParser

* log4net

* UnitsNet

* OsmSharp

* NodaTime

* GMap.Net

* ImageSharp

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.

Another to add

* MassTransit

* AutoFac

* MediatR

* Elsa

* BenchmarkDotNet