ASP.NET Core (which includes MVC) is an open source project under the .NET Foundation. It’s developed at on GitHub at https://github.com/aspnet and continues to build on significant contributions from the community. ASP.NET MVC 5.x, Web API 2.x, and Web Pages 3.x, are also on GitHub at https://github.com/aspnet/aspnetwebstack. 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.
The source code for ASP.NET MVC, ASP.NET Web API and ASP.NET Web Pages (aka Razor) are available under an open source license. Developers outside of Microsoft may submit patches and code contributions that the Microsoft development team will review for potential inclusion. These frameworks continue to be fully supported and staffed by Microsoft.
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.
The PayPal helper for WebMatrix allows you to integrate PayPal payments within your WebMatrix website or e-commerce application. With a few lines of code, you’ll enable your Web site customers to click on a payment button to pay for their purchases with their PayPal accounts.
Rabbit Framework is a lightweight, open-source, CMS-like framework for building dynamic web sites using ASP.NET Web Pages.
The Razor Debugger is a simple web based debugger for ASP.NET WebPages (it might work in MVC but hasn’t really be tested). It started off as an experiment to see if it was possible to have a simple debugging experience for developers using WebMatrix.
The Windows Azure Storage Helper for WebMatrix and ASP.NET Web Pages, allows you to easily retrieve and insert data into Blobs and Tables of Windows Azure Storage Services.
NuGet.org has an extensive directory of open source libraries which you can easily add to your applications via the NuGet package management system.