Docker is a docking application (WindowMaker dock app) which acts as a system tray for KDE and GNOME2. It can be used to replace the panel in either environment, allowing you to have a system tray without running the KDE/GNOME panel or environment.
A notification area (system tray) dockapp.
wmsystray is a program that provides a system tray for your X11 environment. wmsystray can be (and currently works best as) a Window Maker dock app.