awesome-ui

Imitates Windows 7 with JavaFX.

Imitates OS X with JavaFX.

JavaFX Flat UI style.

The better Swing.

Bootstrap 2 & 3 theme for JavaFX.

Provides all JavaFX Controls with the style of Material Design.

Android's Guideline, also refers to the

Guideline for the Apple Watch.

General UI Guideline written by the Eclipse Team.

Guideline for Elementary OS.

The Flat UI guideline written by the maker of

GNOMEs UI Guideline.

A multi-platform .NET UI framework.

.NET UI themes

Material Design for WPF.

Material Design for WinForms.

Create UIs which run on all platforms

Telerik has lot's of components for .NET

Bootstrap Editor and Builder

Bootstrap Editor and Builder

Bootstrap Editor and Builder

Bootstrap Editor and Builder

Build charts like a pro.

Elegant iOS forms.

Provides you with modals and other customizable controls.

Implement FontAwesome in Swift.

Use the Material Icons in Swift.

Extension Sheet Controller.

Coolors is a fast and easy tool that let you generate beautiful color palettes.

Color picker for flat colors.

Pretty much the same.

List of the most common flat colors.

List of all colors following the Material Design concept.

Colorpalette used in metro style UIs.

Much wow, much fonts.

Font search engine.

Lot's of fonts here. Also has tools such as a font generator and font identifier.

Big list of 800+ fonts hosted by Google.

100+ icons in here.

Fontawesome, the most popular iconfont with 600+ icons.

Font with 250+ icons.

200+ icons.

The official Material Design icons.