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*.

WWW: http://alltray.sourceforge.net/