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 .

Alltray running on KDE: Showing:
kcalc, konsole, konqueror, kmail, xchat, kdict

::: Download/Changelog ::: 

Download Page (Linux)
Changelog

::: Copyright/Licence/Contact ::: 

Copyright (c) Jochen Baier, GPL
(not longer maintained by me,
searching for a new developer (email@jochen-baier.de))