Helps get your gmail chat list under control by hiding offline contacts. And if occasionally you do want to access an offline contact, tools like Stylish make it easy to turn the style on and off.

This is the only user style I know of to hide offline gmail/gchat contacts. There are scripts for it that can run through add-ons like greasemonkey, but that exposes more performance and security issues. A style is lighter weight. However, this particular style is difficult to create due to a combination of 1: CSS not supporting selection based on child nodes and 2: gmail indicating offline status in the HTML via only the status image (not the top element for the contact). Therefore, what could otherwise be a very simple style instead requires careful control over element size/spacing and overflows.

[Update 2010-Sep-02] Improve appearance in Google Chrome.