This is a list I've made after reading this HN thread (and its article). I found the links so useful that I'll probably need to check those later.
Feel free to suggest a library (by opening an issue or creating a pull request).
Ah, and make sure to also check these other great lists:
Open Source .NET libraries that make your life easier
- Abot
- Akka.NET
- Autofac
- AutoMapper
- BCrypt.Net
- ChameleonForms
- CsQuery
- CsvHelper
- Cirqus
- Dapper
- DoddleReport
- Dynamic Data
- Effort
- ELMAH
- EPPlus
- Fasterflect
- Fibber
- Fixie
- FluentAssertions
- FluentMigrator
- FluentScheduler
- FluentValidation
- Flurl
- Formo
- FsCheck
- Glimpse
- Hangfire
- HtmlAgilityPack
- Humanizer
- Hyperletter
- ImageResizer
- Insight.Database
- Jil
- Json.NET
- libsodium
- LINQ to DB
- Logary
- MailKit
- MimeKit
- Nancy
- NFluent
- NLog
- NSubstitute
- OpaqueMail
- Polly
- Postal
- Quartz.NET
- Refit
- Regextra
- Restful Routing
- RestSharp
- Serilog
- ServiceStack
- Shouldly
- Simple.Data
- SpecsFor
- StructureMap
- Suave
- T4MVC
- TinyMapper
- Topshelf
- TuesPechkin
- Units.NET
- Serenity
Abot
C# web crawler built for speed and flexibility.
Akka.NET
Port of Akka actors for .NET.
Autofac
An addictive .NET IoC container.
AutoMapper
A convention-based object-object mapper in .NET.
BCrypt.Net
A .Net port of jBCrypt implemented in C#.
ChameleonForms
This library will shape-shift your forms experience in ASP.NET MVC.
CsQuery
CsQuery is a complete CSS selector engine, HTML parser, and jQuery port for C# and .NET 4.
CsvHelper
Library to help reading and writing CSV files.
Cirqus
d60 event sourcing + CQRS framework.
Dapper
A simple object mapper for .Net.
DoddleReport
DoddleReport generates tabular reports from any IEnumerable datasource.
Dynamic Data
Reactive extensions for collections.
Effort
Effort is a powerful tool that enables a convenient way to create automated tests for Entity Framework based applications.
ELMAH
GitHub Google Code NuGet
Error Logging Modules and Handlers for ASP.NET.
EPPlus
EPPlus is a .net library that reads and writes Excel 2007/2010 files using the Open Office Xml format (xlsx).
Fasterflect
.NET library that improves your productivity and application performance when performing reflection operations. It allows you to perform metadata lookup and reflection invocation intuitively while achieving greater performance than the built-in .NET Reflection.
Fibber
An indiscriminate data generator that will generate random data for all properties in a given class based on the property's type vs. its name.
Fixie
Conventional Testing for .NET.
Fluent Assertions
Fluent Assertions is a set of .NET extension methods that allow you to more naturally specify the expected outcome of a TDD or BDD-style test.
FluentMigrator
Fluent Migrations framework for .net.
FluentScheduler
Task scheduler with fluent interface that runs automated tasks (cron jobs) from your application.
FluentValidation
A small validation library for .NET that uses a fluent interface and lambda expressions for building validation rules.
Flurl
Fluent URL builder and testable HTTP for .NET.
Formo
Formo allows you to use your configuration file as a dynamic object. Turn your web.config or application settings into a rich, dynamic object.
FsCheck
Random Testing for .NET.
Glimpse
The open source diagnostics platform for the web.
Hangfire
Incredibly easy way to perform fire-and-forget, delayed and recurring tasks inside ASP.NET applications.
HtmlAgilityPack
An agile HTML parser that builds a read/write DOM and supports plain XPATH or XSLT allowing you to parse "out of the web" HTML files.
Humanizer
Humanizer meets all your .NET needs for manipulating and displaying strings, enums, dates, times, timespans, numbers and quantities.
Hyperletter
Hyperletter helps you distribute your system, like a mix of WCF and ZeroMQ.
ImageResizer
The official repository for ImageResizer.
Jil
Fast .NET JSON (De)Serializer, Built On Sigil.
Json.NET
Json.NET is a popular high-performance JSON framework for .NET.
Insight.Database
Fast, lightweight .NET micro-ORM.
libsodium
A secure cryptographic library.
LINQ to DB
Linq to database provider.
Logary
Logary is the best logging framework for .Net. It's a high performance, multi-target logging, metric and health-check library for mono and .Net.
MailKit
A cross-platform .NET library for IMAP, POP3, and SMTP.
MimeKit
A cross-platform .NET MIME creation and parser library with support for S/MIME, PGP, DKIM, TNEF and Unix mbox spools.
Nancy
Lightweight, low-ceremony, framework for building HTTP based services on .Net and Mono.
NFluent
NFluent is an ergonomic assertion library which aims to fluent your .NET TDD experience (based on simple Check.That() assertion statements).
NLog
Advanced .NET and Silverlight Logging.
NSubstitute
A friendly substitute for .NET mocking frameworks.
OpaqueMail
.NET e-mail library and proxy supporting IMAP, POP3, and SMTP with S/MIME.
Polly
Polly is a .NET 3.5 / 4.0 / 4.5 / PCL library that allows developers to express transient exception handling policies such as Retry, Retry Forever, Wait and Retry or Circuit Breaker in a fluent manner.
Postal
Email sending for asp.net mvc using the view engine system to render emails.
Quartz.NET
A full-featured, open source job scheduling system that can be used from smallest apps to large scale enterprise systems.
Refit
The automatic type-safe REST library for Xamarin and .NET.
Regextra
Regextra simplifies some tasks typically solved via regex so that you no longer have (problems){2}.
Restful Routing
A rails inspired restful routing api for asp .net mvc.
RestSharp
Simple REST and HTTP API Client for .NET.
Serilog
Simple .NET logging with fully-structured events.
ServiceStack
Thoughtfully architected, obscenely fast, thoroughly enjoyable web services for all.
Shouldly
Should testing for .net - the way Asserting *Should* be!
Simple.Data
A light-weight, dynamic data access component for C# 4.0.
T4MVC
T4MVC is a T4 template for ASP.NET MVC apps that creates strongly typed helpers that eliminate the use of literal strings in many places.
TinyMapper
A quick object-object mapper for .NET.
SpecsFor
SpecsFor is a light-weight Behavior-Driven Development framework that focuses on ease of use for *developers* by minimizing testing friction.
StructureMap
A Dependency Injection/Inversion of Control tool for .NET.
Suave
Suave is a simple web development F# library providing a lightweight web server and a set of combinators to manipulate route flow and task composition.
Topshelf
An easy service hosting framework for building Windows services using .NET.
TuesPechkin
A .NET wrapper for the wkhtmltopdf library with an object-oriented API.
Units.NET
Makes life working with units of measurement just a little bit better.
Serenity
Serenity is a Javascript / .NET application platform which has been built on open source technologies.
React
Typescript
Django
Laravel
Facebook
Microsoft
Google
Alibaba
D3
Tencent