I haven't needed to do this in several years, and I wasn't even sure if it still worked in Leopard or Snow Leopard. Fortunately, it does. This evening I was working on a site comp and I needed a handy color-picker Dashboard Widget to stay visible. Enabling Dashboard developer-mode still works:

defaults write com.apple.dashboard devmode YESkillall -HUP Dock

Dragging a Widget while leaving the Dashboard desktop overlay causes the widget to remain visible, and float above all other windows. Bitchin'.