A collection of guidelines and standards for the most common platforms.
Windows
Windows User Experience Interaction Guidelines (HTML)
Windows User Experience Interaction Guidelines (PDF)
Windows XP User Experience Guidelines (PDF)
Mobile Devices
BlackBerry Smartphones - 6.0 UI Guidelines
iPhone Human Interface Guidelines
Palm User Interface Guidelines
UI Design and Interaction Guide for Windows Phone 7
User Interface Guidelines | Android Developers
User Interface Guidelines – HP Palm Developer Center
Windows Mobile 6.5 Design Guidelines
Mac OS
Apple
Human Interface Guidelines (Mac OS X)
Java
Java Look and Feel Design Guidelines (2nd ed.)
Gnome
GNOME Human Interface Guidelines 2.2
KDE
KDE
User Interface Guidelines