A simple ASP.NET MVC implementation of the NuGet back-end gallery and web APIs.
NHibernate is a mature, open source object-relational mapper for the .NET framework. It is actively developed, fully featured and used in thousands of successful projects.
Incredibly simple real-time web for .NET
Opserver is a monitoring system by the team at Stack Exchange, home of Stack Overflow.
Nancy is a lightweight, low-ceremony, framework for building HTTP based services on .Net and Mono
A simple but effective mini-profiler for .net
Fluent Migrator is a migration framework for .NET much like Ruby Migrations.
git-tfs is a two-way bridge between TFS and git, similar to git-svn.
The AWS SDK for .NET enables .NET developers to easily work with Amazon Web Services.
LibGit2Sharp brings all the might and speed of libgit2, a C native Git implementation, to the managed world of .Net and Mono.
This SDK allows you to build Windows Azure applications that take advantage of Azure scalable cloud computing resources.
The .NET framework. Recently open sourced by Microsoft.
Serilog combines the best features of traditional and structured diagnostic logging in an easy-to-use package.
Cake (C# Make) is a cross platform build automation system.
Humanizer meets all your .NET needs for manipulating and displaying strings, enums, dates, times, timespans, numbers and quantities.