Thunderbird-Tray is a system tray launcher tool for Mozilla Thunderbird, the popular e-mail client. You can use TB-Tray to hide Thunderbird to the tray while you don't need it. When you click the tray icon, TB-Tray will restore Thunderbird. It also has a menu of options for quick access to Thunderbird features. Thunderbird-Tray is freeware (open source) and licensed under the GPL.
) "Thunderbird-Tray - Click to enlarge screenshot")