You may have heard about things like "guidelines for user interfaces" - Sometimes I tend to think that there is no such thing as a design guideline for a better user interface because some applications are just plain unusable for a normal human being.

But there are guidelines for almost everything and I wanted to give an overview:

Microsoft:

• Windows XP Guidelines for Applications
• Windows Vista User Experience Guidelines (direct pdf link)
• Office System 2007 User Interface Design Guidelines
• Guidelines for Keyboard User Interface Design

Apple:

• Apple User Experience Guides Overview
• Apple Human Interface Guidelines
• Apple Web Design Guide (oooold)

Linux:

• KDE Standards User Interface Guidelines
• GNOME Human Interface Guidelines
• Motif Style Guide

interesting ones:

• NASA Usability Toolkit
• Blackberry/RIM UI Engine API
• IBM Object-Oriented Interface Design
• SUN Common Desktop Environment (CDE) Style Guide
• SAP R/3 User Experience