I have used a lot of free .NET libraries, some from Microsoft itself! Which ones have you found the most useful?

Dependency Injection/Inversion of Control

1. Unity Framework - Microsoft
2. StructureMap - Jeremy Miller
3. Castle Windsor
4. NInject
5. Spring Framework
6. Autofac
7. Managed Extensibility Framework

Logging

1. Logging Application Block - Microsoft
2. Log4Net - Apache
3. Error Logging Modules and Handlers(ELMAH)
4. NLog

Compression

1. SharpZipLib
2. DotNetZip
3. YUI Compressor (CSS and JS compression/minification)
4. AjaxMinifier (in other downloads) (JS compression. Also includes MSBuild task)

Ajax

1. Ajax Control Toolkit - Microsoft
2. AJAXNet Pro

Data Mapper

1. XmlDataMapper
2. AutoMapper

ORM

1. NHibernate
2. Castle ActiveRecord
3. Subsonic
4. XmlDataMapper

Charting/Graphics

1. Microsoft Chart Controls for ASP.NET 3.5 SP1
2. Microsoft Chart Controls for Winforms
3. ZedGraph Charting
4. NPlot - Charting for ASP.NET and WinForms

PDF Creators/Generators

1. PDFsharp
2. iTextSharp

Unit Testing/Mocking

1. NUnit
2. Rhino Mocks
3. Moq
4. TypeMock.Net
5. xUnit.net
6. mbUnit
7. Machine.Specifications

Automated Web Testing

1. Selenium
2. Watin

URL Rewriting

1. url rewriter
2. UrlRewriting.Net
3. Url Rewriter and Reverse Proxy - Managed Fusion

Controls

1. Krypton - Free winform controls
2. Source Grid - A Grid control
3. Devexpress - free controls

Unclassified

1. CSLA Framework - Business Objects Framework
2. AForge.net - AI, computer vision, genetic algorithms, machine learning
3. Enterprise Library 4.1 - Logging, Exception Management, Validation, Policy Injection
4. File helpers library
5. C5 Collections - Collections for .NET
6. Quartz.NET - Enterprise Job Scheduler for .NET Platform
7. MiscUtil - Utilities by Jon Skeet
8. Lucene.net - Text indexing and searching
9. Json.NET - Linq over JSON
10. Flee - expression evaluator
11. PostSharp - AOP
12. IKVM - brings the extensive world of Java libraries to .NET.

Title of the question taken from here.

[EDIT]
Please provide links to these free libraries as well. Once we have a huge list of this, it can be arranged in categories! Please do not mention .NET Applications/EXEs here.