Bogus - API to generate test data. I've been a Mockaroo fan for years, but Bogus is an easier, more programmable alternative.
CliWrap. A library to effective launch external processes. This is a great alternative to rolling your own with
ClosedXML - A rational API to read and write Excel that works on top of the obfuscated soup that is Open XML. This library takes the place of the great, but not now fully commercial, EPPlus.
Cocona - A superb console application framework.
Dapper - A rational alternative to Entity Framework. Highly recommended. (Dapper powers StackOverflow's file IO for its 6k-per-second requests.)
FluentEmail - A fluent email API with lots of options.
HTMLAgilityPack - Don't use RegEx to parse HTML, use this library.
Humanizer - "Humanize" values in a number of ways (eg, format a date of less than 24 hours ago to "yesterday").
Microsoft.Extensions.Configuration - Very good application configuration with lots of options.
NewtonSoft.Json - A full-featured Json serializer/deserializer. Not needed as much with .NET 5/6 but a life-saver for .NET Framework 4.x.
NodaTime - A better way to work with dates.
Quartz - A job scheduling library.
Serilog - Good logging package.
SharpZipLib - Programatically zip/unzip files.
ShortId - Generate short, unique ids.
Shouldly/FluentAssertions - Two fluent unit test assertion libraries.
xunit A unit testing framework.
YamlDotNet - I don't always parse Yaml, but when I do I use YamlDotNet.