SystemTrayN is an applet that shows the status of various applications on a computer. It is compatible with the freedesktop.org notification area spec so it can be used with programs compatible with either gnome 2.2 or future versions of KDE. .
To use it, just drag it to your panel. Icons in the system tray can display important messages in a small window near the tray. Click on the window to acknowledge the message.
This is an updated version of Andy Hanton's SystemTray application and it is now available via the ZeroInstall injector.