dnAnalytics is a numerical library for the .NET Framework licensed under the Microsoft Public License. The library is written in C# and is available as a fully managed library, or as a native version that uses Intel® Math Kernel Library (MKL). The native version of dnAnalytics provides significantly better performance when working with large sets of data. dnAnalytics is compatible with .NET 2.0 or later, and Mono. The managed version will run on a Windows XP or newer, and any platform that supports Mono. The native version supports 32bit and 64bit versions of Windows XP or newer, and 32bit and 64bit versions of Linux....

PostSharp Core is a platform for post-compilation enhancement of .NET assemblies. PostSharp Laos is an Aspect-Oriented Programming (AOP) famework the .NET. Using PostSharp Laos, you can design custom attributes that really add new behaviors to your code.

WCell is an open-source server emulation package for the popular MMORPG, World of Warcraft ©. It is written in C#, and utilizes the .NET framework. It employs many powerful technologies such as XML, WCF, and the ActiveRecord pattern. WCell is multi-threaded and thus designed to scale with today's multi-processor servers....

From the folks that brought you DotNetOpenId...

This library (will) bring a full implementation of the OAuth protocol, written in C#.

It's in alpha form right now, due to some impending refactoring that needs to be done to improve usability, but it's quite fully functional now. It's got smart client consumer, web client consumer, web service provider and WCF samples....

Description will follow. Read the team blog for initial information about mixins (extensibility for C# and VB.NET classes) and re-linq (a library for creating LINQ providers).

NAudio is an open source .NET audio and MIDI library, containing dozens of useful audio related classes intended to speed development of audio related utilities in .NET. It has been in development since 2001 and has grown to include a wide variety of features. While some parts of the library are relatively new and incomplete, the more mature features have undergone extensive testing and can be quickly used to add audio capabilities to an existing .NET application....

Irony is a development kit for implementing languages on .NET platform. Unlike most existing yacc/lex-style solutions Irony does not employ any scanner or parser code generation from grammar specifications written in a specialized meta-language. In Irony the target language grammar is coded directly in c# using operator overloading to express grammar constructs. Irony's scanner and parser modules use the grammar encoded as c# class to control the parsing process. See the expression grammar sample for an example of grammar definition in c# class, and using it in a working parser....

NetTopologySuite is a C#/.NET port of JTS Topology Suite, a Java library for GIS operations, (OpenGIS compliant).
The main goal of this project is to have a .NET GIS solution that is fast and reliable for any kind of .NET platform, PocketPC and SQL Server 2005 (through CLR integration) included....

The Unity Application Block (Unity) is a lightweight extensible dependency injection container with support for constructor, property, and method call injection.

Appcelerator is an open source platform for building rich Internet applications (RIA).