UI Style Guides

Android User Interface Guidelines

• Android UI Guidelines
• Also see Google Material Design guidelines
• Android Ice Cream Sandwich User Interface Guidelines
• Google Glass UI guidelines
• Google Cast (Chromecast) UX guidelines (new Dec 2013)

Apple iPhone User Interface Guidelines

• iPhone GUI PSD
• iPhone interface PSD
• iOS 9 Human Interface Guidelines
• iOS 10 UI Guidelines- What's New

Apple iPad User Interface Guidelines

• iPad SDK / iPad Human Interface Usability Guidelines(requires membership)
• see: iPad User Experience Guidelines

Other Mobile Usability Guidelines

• Nokia Top 10 Usability Guidelines for Games
• W3C Mobile Web Best Practices 1.0 User Interface Guidelines (2008)
• Windows Phone 8 UI Guidelines
• Ubuntu Mobile User Interface Guidelines
• Palm webOS for Palm Pre
• Samsung Bada User Interface Style Guide

Tablet PC User Interface Guidelines

• The Challenge of Designing Interfaces for the Tablet PC (WayBackWeb archived)
• Eight Essentials of Tablet User Interface Success (WayBackWeb archived)
• Eight More Rules of Tablet UI Success (WayBackWeb archived)

Apple (iOS) Usability Guidelines

• Apple Human Interface Guidelines for iOS
• Apple User Experience Guides

Microsoft Windows Usability Guidelines

• Windows 8 UX guidelines for Windows Store Apps
• Windows 10 UI Style Guidelines

Open Source Usability Guidelines

• Motif Style Guide
• KDE User Interface Guidelines
• Gnome UI Library
• Gnome Human Interface Guidelines 2.2.1
• Ubuntu 10.04 (Lucid Lynx) "Guide" Wiki

Accessibility Guidelines

• Introduction to Web Accessibility
• W3C WCAG: Web Content Accessibility Guidelines 2.0
• Section 508 Checklist
• W3C ARIA Accessibility Rich Internet Application guidelines
• Adobe PDF Accessibility (PDF)
• Material Design Accessibility Guidelines
• Game Accessibility Guidelines

AJAX and Rich Internet Application (RIA) related Usability Guidelines (What is Ajax?)

• More Ajax Usability Guidelines
• Ten good practices for writing (AJAX) JavaScript in 2005
• AJAX Usability Guidelines and Ajax Patterns

Sun Microsystems- Java related

• Java Look and Feel Guidelines (PDF) 
• Java Look and Feel Guidelines (BOOK)

Java Swing related UI style guides

• A Java Swing GUI Framework
• Using Java Swing's Pluggable Look and Feel

VR, AR, Mixed Reality UX guidelines

• Google Daydream and Cardboard VR UX Guidelines
• Occulus VR UI Guidelines
• Hololens: Microsoft Holographic - Mixed Reality UI Guidelines
• Designing for Mixed Reality (Hololens) UX guidelines for Mixed Reality
• Microsoft Kinect for Windows Human Interface Guidelines v1.8.0 UX Guidelines (PDF shortcut)
• Microsoft Surface User Experience Guidelines
• Leap Motion UX Guidelines
• Unreal Engine VR UX Guidelines
• Unity VR UX Guidelines

Corporate User Interface Standards and Guidelines (samples)

• Telstra Online Standards (WayBackWeb archived)
• Ameritech Graphical User Interface Standards and Design Guidelines (WayBackWeb archived)

Government funded Usability Guidelines

• MITRE Guidelines for Designing User Interface Software (US Airforce)
• Research based Web Design and Usability Guidelines (PDF) (US Dept. of Health and Human Services)
• Usability.gov - Usability Guidelines

Other Usability and UI Guidelines

• Silicon Graphics Indigo Magic User Interface Guidelines
• Smith and Mosier HCI Guidelines (PDF) (1984, yet amazingly pertinent still today)
• User Interface Design and Usability (links)