Microsoft continues its commitment to open source. We not only host projects at the CodePlex forge and support many open source projects, but we also create our own open-source projects that the community can improve and extend. In addition to Microsoft-supported open source projects, there are thousands of great open source projects, applications, and libraries that you can use with ASP.NET today. Here you'll find a few of our favorites. If you're looking for Samples, there's lots of samples here as well as on http://code.msdn.microsoft.com. We'll update these pages as we come upon other cool projects. - Jon Galloway and Scott Hanselman
Juice UI is a collection of WebForms components that brings jQuery UI Widgets to your project with ease.
At its core Glimpse allows you to debug your web site or web service right in the browser. Glimpse allows you to "Glimpse" into what's going on in your web server. In other words what Firebug is to debugging your client side code, Glimpse is to debugging your server within the client.
Write regular CSS with your .NET apps, then add a few variables, mixins and nested rules. “.less” (pronounced dot-less) is a .NET port of the funky ruby LESS library.
DotNetOpenAuth is a well-established open source library that bring OpenID, OAuth, and ICard capabilities to the Microsoft .NET Framework.
The ASP.NET Sprite and Image Optimization framework is designed to decrease the amount of time required to request and display a page from a web server by performing a variety of optimizations on the page’s images.
ELMAH (Error Logging Modules and Handlers) is an application-wide error logging facility that is completely pluggable. It can be dynamically added to a running ASP.NET web application, or even all ASP.NET web applications on a machine, without any need for re-compilation or re-deployment.
A simple but effective mini-profiler for ASP.NET MVC and ASP.NET. It does not attach itself to every single method call. Instead, it provides: an ADO.NET profiler and a pragmatic Step instrumentation that you can add to code you want to explicitly profile.
Massive is a "wrapper" for your DB tables and uses System.Dynamic extensively. Drop the code file into your app and change it as you wish. It’s a micro-ORM that hides some things and embraces others.
Dapper is a single file you can drop in to your project that will extend your IDbConnection interface. It’s a “micro-ORM” for people who love SQL.
N2 is a lightweight CMS framework. With just a few strokes of your keyboard a strongly typed model emerges complete with a management UI.
A simple Model-View-Presenter framework for ASP.NET Web Forms to aid in building testable and maintainable ASP.NET projects.
The Ajax Control Toolkit contains a rich set of controls that you can use to build highly responsive and interactive Ajax-enabled Web applications. The Ajax Control Toolkit contains more than 40 controls, including the AutoComplete, CollapsiblePanel, ColorPicker, MaskedEdit, Calendar, Accordion, HTML Editor Extender, and Watermark controls. Using the Ajax Control Toolkit, you can build Ajax-enabled ASP.NET Web Forms applications by dragging-and-dropping Toolkit controls from the Visual Studio Toolbox onto an ASP.NET Web Forms page.
The ASP.NET Sprite Web Forms control allows you to use sprites and image inlining in ASP.NET 4 Web Forms projects.
CKEditor is a text editor to be used inside web pages. It's a WYSIWYG editor; the text being edited looks as similar as possible to the results users have when publishing it. It brings to the web common editing features found on desktop editing applications.
FreeTextBox is the most-used HTML editor for ASP.NET. It is compatible with IE on the PC, and Mozilla and Firefox on all platforms. It is used in major Open Source projects as well as excellent packages like Smarter Mail.
A set of libraries, tools, and utilities for .NET developers to secure their applications including site spam filtering services, CAPTCHA controls, malware & phishing filter, and customizable password generator.
NuGet.org has an extensive directory of open source libraries which you can easily add to your applications via the NuGet package management system.
Composite C1 is a developer friendly Web Content Mangement System (CMS) aimed at marketing & communication professionals who need to easily manage corporate website content with a consistent visual identity