Exploring the .NET Open Source Application 2026

This blog post is a summary of a talk I gave at the .NET Thailand Developer Day 2026 at Seven Peaks, 5F The PARQ, Bangkok.

The idea for this topic came from attending FOSSASIA 2026 Community Day, where I listened to a talk titled "Recommend Open Source apps to a new user." This got me thinking about the .NET side — ever since .NET became open source back in 2017, now in 2026, let's take a look at what has grown beyond just libraries. We've seen many great ones such as RestSharp / Serilog (Logging) / NUnit (Testing) / OpenTelemetry / Insight.Database / Dapper / FluentValidation / Hangfire (Background Job) / Polly (Resilience and Transient-Fault-Handling)

but now let's explore the ready-to-use applications available across various domains, I categorized into 5 main groups

Exploring the .NET Open Source Application 2026

1. Productivity & Desktop Utilities

Applications designed to streamline your daily workflows and supercharge your desktop experience.

• ShareX: Much more than a screenshot tool. It allows you to capture scrolling windows, record your screen, extract text from images (OCR), and set up automated workflows to upload files directly to dozens of cloud services.
• Files: A modern, beautifully designed alternative to the native Windows File Explorer. It features a tabbed interface, a dual-pane layout, custom themes, and seamless cloud storage integration.
• PowerToys: A set of system utilities developed in collaboration with Microsoft for power users. It includes tools like FancyZones for complex window snapping, PowerToys Run for lightning-fast app launching, and a system-wide color picker.
• QuickLook: Brings the macOS "Spacebar to preview" functionality to Windows. You can instantly view PDFs, images, text files, and videos without waiting for heavy applications to load.
• EarTrumpet: An advanced, intuitive volume control app for Windows that lets you manage audio levels and outputs for individual applications effortlessly.
• ScreenToGif: A lightweight and incredibly useful tool to record your screen, webcam, or digital sketchboard. It includes a built-in frame-by-frame editor to export high-quality GIFs or videos.
• Fluent Search: A fast, keyboard-centric search engine for Windows that lets you find files, launch apps, and even switch between browser tabs without ever touching your mouse.

2. Media & Entertainment

Self-hosted solutions and tools tailored for your entertainment, gaming, and media consumption.

• Jellyfin: A completely free software media system that puts you in control. It allows you to host, manage, and stream your own movies, TV shows, music, and EBook libraries across all your devices without any tracking or premium licenses.
• Kavita: A blazing-fast, feature-rich reading server built from the ground up for manga, comics, and novels. It features a great responsive reader and supports text highlighting for EPUB files.
• SteamTools: A handy utility toolkit designed to enhance and manage your game clients, optimizing storage and settings across platforms like Steam, Epic Games, and Uplay.
• osu!: A highly popular, free-to-play rhythm game built with C# and the .NET framework, boasting a massive, community-driven beatmap library and a highly competitive scene.
• Playnite: An open-source video game library manager that provides one unified, customizable interface for all your games across platforms like Steam, Epic, GOG, and even emulated console games.
• Ryujinx: An experimental, highly accurate Nintendo Switch emulator written entirely in C# that offers excellent performance for gaming on PC.

3. Creative & Design

Accessible yet powerful tools tailored for visual editing, drawing, and writing.

• Pinta: A free, open-source program for drawing and image editing. Inspired by Paint.NET, it offers unlimited undo history and multi-layer support for quick, hassle-free edits across Linux, Mac, and Windows.
• Paint.NET: A classic, highly capable image and photo editing software for Windows. It features an intuitive user interface with support for layers, special effects, and a wide variety of powerful editing tools.
• Markdown Monster: A robust Markdown editor and viewer that lets you write with syntax highlighting and an instant HTML preview. It also includes built-in tools to publish your writing directly to platforms like WordPress or Medium.
• HandBrake (WinApp): The Windows graphical user interface for the popular multi-threaded video transcoder. It makes converting video from nearly any format to modern, widely supported codecs incredibly simple.

4. Developer Tools

Essential helpers and automation tools to make software development faster and easier.

• DevToys: Often called the "Swiss Army knife" for developers, it works entirely offline to provide dozens of tiny, essential tools in one place—such as JSON formatters, Base64 encoders/decoders, text diff checkers, and JWT decoders.
• CSharpRepl: A cross-platform command-line REPL (Read-Eval-Print Loop) for C#. It supports syntax highlighting and lets you load NuGet packages directly into your session for rapid prototyping and API testing.
• dotnet-please: A handy CLI tool designed to help maintain clean .NET repositories by consolidating NuGet packages, managing centralized versioning, and effortlessly cleaning up project files.
• mockaco: An HTTP API mocking server that is heavily configurable using purely JSON files. It’s perfect for frontend developers who need a fake backend to test against quickly.
• Cake (C# Make): A cross-platform build automation system with a C# DSL (Domain Specific Language) that lets you write your CI/CD build scripts in the language you already know.
• Papercut-SMTP: A simplified local SMTP server designed for development. It catches and displays emails sent by your local apps, ensuring test emails never accidentally reach real users.
• ILSpy: The industry-standard open-source .NET assembly browser and decompiler, essential for reverse engineering and debugging compiled code.
• GitExtensions & GitVersion: GitExtensions provides a powerful graphical user interface for Git on Windows, while GitVersion automatically generates semantic version numbers based on your Git commit history.
• ReportGenerator: A tool that takes code coverage reports from testing frameworks and converts them into highly readable, visual HTML reports.
• dotnet-outdated: A .NET Core global tool to display and update outdated NuGet packages in a project

5. Server & Infrastructure

Robust, enterprise-grade backend systems and frameworks running securely on .NET.

• Bitwarden (Server): The core backend for the popular open-source password manager. It handles secure secret management, encrypted vault syncing, and cryptographic operations.
• nopCommerce: A highly customizable, full-featured enterprise e-commerce platform built on ASP.NET Core, powering thousands of online stores worldwide.
• Squidex: A headless Content Management System (CMS) with a rich API, allowing developers to consume content seamlessly while editors manage data independently of the frontend presentation.
• Umbraco CMS & Orchard Core: Two of the most highly regarded open-source .NET CMS platforms. Umbraco is known for its friendly editing experience, while Orchard Core offers a modular, multi-tenant application framework.
• Cofoundry: provides an extensible .NET Core CMS specifically focused on a great developer experience.
• BTCPay Server: A self-hosted, open-source cryptocurrency payment processor that allows merchants to accept Bitcoin and other cryptos securely without fees or intermediaries.
• simplcommerce: offers a modular, cross-platform e-commerce framework (Modular Monolith Design)
• Jackett: Works as a proxy server that translates queries from media automation apps (like Sonarr or Radarr) into tracker-site-specific HTTP queries, streamlining your media downloading process.
• Notesnook Sync Server: The backend synchronization server for Notesnook, an open-source, end-to-end encrypted note-taking application designed as a privacy-first alternative to Evernote.

Why Should We Adopt Open Source?

📌User & Organization View:

• Cost-Effective: Lowers the Total Cost of Ownership (TCO) and saves on expensive licensing fees, as most tools are free or highly affordable.
• User Experience (UX): Open-source tools increasingly focus on intuitive interfaces, leading to higher employee adoption and daily usability.
• Productivity: Helps teams get work done faster, driving business agility and velocity.
• Ready to Use: Rapid deployment means you can download, install, and start working instantly.
• Privacy & Control: Self-hosted options ensure strict compliance, strong data governance, and the protection of personal data.

📌Developer View:

Reading and contributing to .NET open-source projects is one of the best ways to level up your engineering skills:

• Improve Your Logic: Learn how to write defensive code, manage complex thread-safety (concurrency), and implement high-performance techniques.
• Understand Patterns: Study real-world design patterns, scalable software architectures, and CI/CD automation (DevOps) directly from production-ready codebases.
• Enhance Your Skillset: Improve your ability to debug complex issues, write better automated tests, and communicate intent through "Code as Documentation."

📌How to Contribute?

You don't have to be a senior C# developer to give back to the community!

• Act as a Tester: Use the software and report bugs on GitHub.
• Update Documentation: Fix typos, write tutorials, or translate the UI into your local language.
• Provide Funding: Support your favorite maintainers financially through platforms like GitHub Sponsors or Patreon.
• Start Coding: Submit Pull Requests to fix bugs, update outdated NuGet libraries, or improve CI/CD pipelines.

Slide For my speak