Although by default widgets are confined to the Dashboard layer, a widget can be dragged from the widget bar to the desktop by simply selecting the widget, dragging it, and then switching back to the Desktop, from the Dashboard, before dropping the widget. The widget will remain floating on the desktop until the next time the Dashboard is opened.
To keep one or more widgets on the desktop in a more permanent fashion, the Dashboard "devmode" must be activated. Enter the following into the Terminal, or Terminal.app (/Applications/Utilities/Terminal.app):defaults write com.apple.dashboard devmode YES
and then restart the Dock (and Dashboard):killall Dock
Once this option has been set, widgets dragged from the Dashboard will remain floating on the desktop, even after log out or shutdown. To move a desktop widget back to the Dashboard, simply reverse the process used to move it onto the desktop.
Another option for widgets on the desktop is to use the shareware utility Amnesty Widget Browser, a dashboard emulator that also allows the user to select which level (desktop, standard or floating) a widget occupies while it resides on the desktop.