Human User Interface guidelines
Appearance
Human User Interface guidelines are a list of rules that applications should adhere to, in order to share a common look and feel with other applications in a particular desktop environment.
External Links:
GNOME http://developer.gnome.org/projects/gup/hig/
KDE http://developer.kde.org/documentation/standards/kde/style/basics/
Apple OSX http://developer.apple.com/documentation/mac/HIGuidelines/HIGuidelines-2.html
Microsoft Windows XP http://www.microsoft.com/whdc/hwdev/windowsxp/downloads/default.mspx