CodeTextBox

•    CSharp

CodeTextBox is a Windows Forms control for colorizing code while you are typing in the text box.

Microsoft All-In-One Code Framework - a centralized code sample library

•    DotNet

The Microsoft All-In-One Code Framework is a centralized scenario-focused code sample library provided by the Microsoft Community team.

Extended WPF Toolkit™ Community Edition

•    WPF

The Extended WPF Toolkit™ is the number one collection of WPF controls, components and utilities for creating next generation Windows applications.

Silverlight Toolkit

•    Silverlight

Silverlight Toolkit and Silverlight for Windows Phone Toolkit are a set of Microsoft components for developers.

DevExtreme - HTML5 JavaScript Component Suite for Responsive Web Development

•    Javascript

Voted the best cross-platform component suite containing everything you need to create responsive web apps for touch devices and traditional desktops: data grid, interactive charts, data editors, navigation and multi-purpose widgets that are designed to look great and provide powerful functionality in any browser.

elemental - A flexible and beautiful UI framework for React.js

•    Javascript

React & CSS UI Framework. Currently under development; built by @JedWatson and @JossMac at Thinkmill.

Windows Presentation Foundation (WPF)

•    WPF

This is the main site for updates on the WPF roadmap and the portal for accessing the WPF Toolkit and the WPF Futures releases.

wpftoolkit - All the controls missing in WPF. Over 1 million downloads.

•    CSharp

Extended WPF Toolkit™ by Xceed is the number one collection of WPF controls, components and utilities for creating next generation Windows applications. Use it to build professional looking, modern, and easy to use WPF applications. It has been downloaded more than 1 million times here, on Codeplex, and on NuGet.This free and open source toolkit is provided under the Microsoft Public License. A summary can be found on tl;drLegal.

ui-fabric-ios - Office UI Fabric for iOS - The front-end framework for building experiences for Office and Office 365

•    Swift

Office UI Fabric for iOS is a native library that provides the Office UI experience for the iOS platform. It contains information about colors and typography, as well as custom controls and customizations for platform controls, all from the official Fluent design language used in Office and Office 365 products. Office UI Fabric for iOS provides colors and typography core to experiences within the Fluent Design language.

The Windows Phone Toolkit

•    Silverlight

A collection of standard controls, animations and helpers for creating beautiful and consistent WP7 & WP8 apps

AvalonDock

•    WPF

AvalonDock is a WPF control for adding a docking layout system to your application.

Grid.Mvc

Grid.Mvc - is a component that allows you easy construction of HTML tables for displaying, paging and sorting data from a collection of Model objects.

SpeechKITT - 🗣 A flexible GUI for Speech Recognition

•    Javascript

Speech KITT makes it easy to add a GUI to sites using Speech Recognition. Whether you are using annyang, a different library or webkitSpeechRecognition directly, KITT will take care of the GUI. Speech KITT provides a graphical interface for the user to start or stop Speech Recognition and see its current status. It can also help guide the user on how to interact with your site using their voice, providing instructions and sample commands. It can even be used to carry a natural conversation with the user, asking questions the user can answer with his voice, and then asking follow up questions.

react-native-track-player - A fully fledged audio module created for music apps

•    Java

Check Platform Support for more information. After trying to team up modules like react-native-sound, react-native-music-controls and react-native-google-cast, I've noticed that their structure and the way should be tied together can cause a lot problems (mainly on Android). Those can heavily affect the app stability and user experience.

Fluid - Windows Mobile .NET Touch Controls

•    DotNet

Windows Mobile Fluid Touch Controls Library Fluid is a .NET 2.0 control library for Windows Mobile 6.0/6.1 with touch controls.

Nice Libraries

Nice libraries offers you a set of different custom controls, custom dialogs and API functions.

Silvermoon

•    DotNet

.NET Compact Framework 3.5 Control library with OpenGL controls to build WP7 like apps on WM6.x devices. The apps are completely stylable with style dictionaries to change the UI e.g. from WP7 to anroid or iOS or even any imaginable style.

Simple but Cool Silverlight Message Boxes (Info, Error, Confirm, etc.)

•    Silverlight

Simple but presentable message boxes for Silverlight developers. Designed for ease of integration with existing projects.

FlipControl 3D

•    WPF

This is a 3D Flip Control for WPF. It has a FrontSide and a BackSide. On Mouse Enter you can see the Backside.

PickList (Dual Listbox) ASP.NET WebControl

•    ASPNET

Renders two listboxes side-by-side with buttons between them form moving options from one listbox to the other. Other names for this type of control include dual listbox and double listbox.