With AllTray you can dock any application with no native tray icon (like Evolution, Thunderbird, Terminals) into the system tray. A high-light feature is that a click on the "close" button will minimize back to system tray. It works well with GNOME, KDE, Xfce 4*, Fluxbox* and WindowMaker*. (*) No drag 'n drop support. Enable with "-nm" option.