60+ .NET libraries every developer should know about.
- Yedda Twitter Library
- FileHelpers Library
- Elmah
- Log4Net
- Enterprise DT FTP Library
- HtmlAgilityPack
- OpenAuth Library
- Microsoft AntiXSS library
- C5 Collections – Collections for .NET
Dependency Injection/Inversion of Control
- Unity Framework – Microsoft
- StructureMap – Jeremy Miller
- Castle Windsor
- NInject
- Spring Framework
- Autofac
- Managed Extensibility Framework
Logging
- Logging Application Block – Microsoft
- NLog
Compression
- SharpZipLib
- DotNetZip
- YUI Compressor (CSS and JS compression/minification)
Ajax
- Ajax Control Toolkit – Microsoft
- AJAXNet Pro
Data Mapper
ORM
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
PDF Creators/Generators
Unit Testing/Mocking
Automated Web Testing
URL Rewriting
- url rewriter
- UrlRewriting.Net
- Url Rewriter and Reverse Proxy – Managed Fusion
Controls
- Krypton – Free winform controls
- Source Grid – A Grid control
- Devexpress – free controls
Unclassified
- CSLA Framework – Business Objects Framework
- AForge.net – AI, computer vision, genetic algorithms, machine learning
- Enterprise Library 4.1 – Logging, Exception Management, Validation, Policy Injection
- Quartz.NET – Enterprise Job Scheduler for .NET Platform
- MiscUtil – Utilities by Jon Skeet
- 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.
- FlickrNET – Flickr library for .NET
- PowerCollections – Power Collections for .NET
- Facebook.net – Facebook API interface.

Another to add would be the ADO.NET SQLite provider. If you have never used SQLite, it is a full relational DBMS contained in a single tiny .dll. I recently wrote a post about it at my blog, you should check it out!
http://brennydoogles.wordpress.com/2010/02/26/using-sqlite-with-c-2/
By the way, PDFsharp has moved to http://www.pdfsharp.net/Default.aspx?AspxAutoDetectCookieSupport=1