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*
I have set up a key shortcut in fluxbox, so I only have to press one button and click the window i want to send to tray. Comes in handy! Especially useful for thunderbird.