The main purpose of RestSharp is to make synchronous and asynchronous calls to remote resources over HTTP. As the name suggests, the main audience of RestSharp are developers who use REST APIs.

An advanced, composable, functional reactive model-view-viewmodel framework for all .NET platforms that is inspired by functional reactive programming. ReactiveUI allows you to abstract mutable state away from your user interfaces, express the idea around a feature in one readable place and improve the testability of your application.

Reactive Extensions is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators. Using Rx, developers represent asynchronous data streams with Observables, query asynchronous data streams using LINQ operators, and parameterize the concurrency in the asynchronous data streams using Schedulers. Simply put, Rx = Observables + LINQ + Schedulers.

React Native for Windows is a framework developed by Facebook that enables you to build world-class application experiences on native platforms using a consistent developer experience based on JavaScript and React. The focus of React Native is on developer efficiency across all the platforms you care about - learn once, write anywhere.

Python.NET is a package that gives Python programmers nearly seamless integration with the .NET 4.0+ Common Language Runtime (CLR) on Windows and Mono runtime on Linux and OSX. Python for .NET provides a powerful application scripting tool for .NET developers. Using this package you can script .NET applications or build entire applications in Python, using .NET services and components written in any language that targets the CLR (C#, VB.NET, F#, C++/CLI).

Polly is a .NET resilience and transient-fault-handling library that allows developers to express policies such as Retry, Circuit Breaker, Timeout, Bulkhead Isolation, and Fallback in a fluent and thread-safe manner. From version 6.0.1, Polly targets .NET Standard 1.1 and 2.0+.

Piranha CMS is a lightweight and unobtrusive CMS Library for .NET 6. It can be used to add CMS functionality to your existing application or to build a new website from scratch. Piranha CMS is built on a modern and future proof technology stack that ensures that both developers and content editors will get the best experience possible.

A repo containing all P/Invoke code so you don't have to import it every time. Ready in consumable NuGet packages. Maintained and updated to support the latest Windows OS.

Peachpie is a modern compiler and runtime of PHP for .NET and .NET Core, built on top of Roslyn. It aims to be fully compatible with the PHP language, including PHP 7.1 and any newly released versions.

Katana is a flexible set of components for building and hosting OWIN-based web applications on .NET Framework. This repo is the home for the Katana host, server, and middleware source code and documentation. Official releases of Katana components (including prerelease versions) can be found on https://nuget.org. These products are developed by the ASP.NET team at Microsoft in collaboration with a community of open source developers.

Orleans is a framework that provides a straight-forward approach to building distributed high-scale computing applications, without the need to learn and apply complex concurrency or other scaling patterns. It was created by Microsoft Research and designed for use in the cloud. Orleans has been used extensively running in Microsoft Azure by several Microsoft product groups, most notably by 343 Industries as a platform for all of Halo 4 and Halo 5 cloud services, as well as by a number of other projects and companies.

Orchard Core Orchard Core is an open-source modular and multi-tenant application framework built with ASP.NET Core, and a content management system (CMS) built on top of that application framework. All components in Orchard Core can be replaced or extended. Content is built from easily composable building blocks. Modules extend the system in a very decoupled fashion, where a commenting module for example can as easily apply to pages, blog posts, photos or products. A rich UI composition system completes the picture and ensures that you can get the exact presentation that you need for your content.

Orchard is a free, open source, community-focused Content Management System built on the ASP.NET MVC platform.

Oqtane is a modular application framework built natively on .NET Core and Blazor. It supports multi-tenancy, a fully dynamic page compositing model, designer friendly themes, extensibility via third party modules, and an intuitive relational data model.

The Open XML SDK provides .NET developers with a set of strongly typed classes that make it easy to read, write and manipulate the parts and content in an Open XML document such as the DOCX, XLSX or PPTX files created by Microsoft Office. It can be used in any .NET solution, and work is underway to enable use of it from Mono as well.

Open RIA Services is a framework for helping with the development of rich internet connected native "n-tier" applications. It is the evolved Open Source version of WCF RIA Services.

Open Live Writer makes it easy to write, preview, and post to your blog.

OmniSharp is a family of Open Source projects, each with one goal: To enable a great .NET experience in YOUR editor of choice.

OData Connected Service is Microsoft visual studio extension which allows users to generate proxy classes from the OData Metadata.

NUnit is a unit testing framework for C#, F#, VB.NET and other .NET languages.