Package managers for external libraries
•NuGet (formerly known as NuPack) – Microsoft (developer-focused package management system for the .NET platform intent on simplifying the process of incorporating third party libraries into a .NET application during development)
•OpenWrap – Sebastien Lambla – Open Source Dependency Manager for .net applications

Build Tools
•Prebuild – Generate project files for all VS version, including major IDE’s and tools like SharpDevelop, MonoDevelop, NAnt and Autotools

Dependency Injection/Inversion of Control
•Unity Framework – Microsoft
•StructureMap – Jeremy Miller
•Castle Windsor
•NInject
•Spring Framework
•Autofac
•Simple Injector
•Managed Extensibility Framework
•TinyIoC

Logging
•Logging Application Block – Microsoft
•Log4Net – Apache
•Error Logging Modules and Handlers(ELMAH)
•NLog
•CuttingEdge.Logging
•Logging Abstraction Api “Common.Logging”

Validation
•Validation Application Block – Microsoft
•xVal
•Fluent Validation

Design by Contract
•Microsoft Code Contracts
•LinFu
•CuttingEdge.Conditions

Compression
•SharpZipLib
•DotNetZip
•YUI Compressor (CSS and JS minification)
•AjaxMinifier (in other downloads) (CSS and JS minification. Also includes MSBuild task)
•SevenZipSharp to pack, unpack a lot of different packages (including RAR, 7ZIP, etc).
•ZipStorer A Pure C# Class to Store Files in Zip

Ajax
•Ajax Control Toolkit – Microsoft
•AJAXNet Pro
•ASP.NET MVC Project Awesome – ajax helpers for asp.net mvc

Data Mapper
•XmlDataMapper
•AutoMapper
•Dapper
•Massive
•Data accessors from the Data Access Application Block
•ValueInjecter

ORM
•NHibernate and FluentNHibernate
•Castle ActiveRecord
•Subsonic
•XmlDataMapper
•Truss Data binding and real time property sync library

Charting/Graphics
•Microsoft Chart Controls for ASP.NET 3.5 SP1
•Microsoft Chart Controls for Winforms
•ZedGraph Charting
•NPlot – Charting for ASP.NET and WinForms
•ImageResizer – Url-driven image processing for both ASP.NET and .NET

PDF Creators/Generators
•PDFsharp
•iTextSharp

Unit Testing/Mocking
•NUnit
•Rhino Mocks
•Moq
•TypeMock.Net
•xUnit.net
•Gallio/MbUnit
•Machine.Specifications

Automated Web Testing
•Selenium
•Watin

Misc Testing/Quality Support/Behavior Driven Development (BDD)
•Testdatagenerator NBuilder
•BDD StoryQ
•BDD specflow

URL Rewriting
•url rewriter
•UrlRewriting.Net
•Url Rewriter and Reverse Proxy – Managed Fusion/Nick Berardi

Web Debugging
•Glimpse – Firebug for your webserver

Controls
•Krypton – Free winform controls
•Source Grid – A Grid control

MS Word/Excel Documents Manipulation
•DocX to create, read, manipulate formatted word documents. Easy syntax, working nicely, actively developed. No Microsoft Office necessary.
•Excel XML Writer allows creation of .XLS (Excel) files. No Microsoft Office necessary. Been a while since it has been updated. It also provides code generator to create code from already created XLS file (saved as xml). Haven’t tested this but looks very promising. Too bad author is long time gone.
•Excel Reader allows creation/reading of .XLS (Excel) files. No Microsoft Office necessary. Been a while since it has been updated.
•Excel Package allows creation/reading of .XLSX (Excel 2007) files. No Microsoft Office necessary. Author is gone so it’s out of date.
•EPPlus is based on Excel Package and allows creation/reading of .XLSX (Excel 2007). It is actually the most advanced even comparing to NPOI.
•NPOI is the .NET version of POI Java project at http://poi.apache.org/. POI is an open source project which can help you read/write xls, doc, ppt files.

Social Media
•LinqToTwitter – Linq-based wrapper for all Twitter API functionality in C#
•Facebook C# SDK – A toolkit for creating facebook applications / integrating websites with Facebook using the new Graph API or the old rest API.

Serialisation
•sharpserializer – xml/binary serializer for wpf, asp.net and silverlight
•protobuf-net – .NET implementation of google’s cross-platform binary serializer (for all .NET platforms)

Unclassified
•CSLA Framework – Business Objects Framework
•AForge.net – AI, computer vision, genetic algorithms, machine learning
•Prism – Composit UI application block for WPF, Silverlight and WP7 – Microsoft patterns & practices
•Enterprise Library 5.0 – Logging, Exception Management, Caching, Cryptography, Data Access, Validation, Security, Policy Injection – Microsoft patterns & practices
•File helpers library
•C5 Collections – Collections for .NET
•Quartz.NET – Enterprise Job Scheduler for .NET Platform
•MiscUtil – Utilities by Jon Skeet
•Noda Time – DateTime replacement (idomatic port of Joda Time from Java)
•Lucene.net – Text indexing and searching
•Json.NET – Linq over JSON
•Flee – expression evaluator
•PostSharp – AOP
•IKVM – brings the extensive world of Java libraries to .NET.
•C# Webserver – Embeddable webserver
•Long Path – Microsoft
•.NET Engines for the GOLD Parsing System
•Smart Thread Pool – Thread Pool management library
•NCQRS – library for event-driven architectures (CQRS).
•NBug – Automated exception, error reporting tool (can generate minidumps)

[출처] [C#] 무료로 제공하는 .NET 라이브러리 리스트|작성자 벤자민