I use a small utility called Witch to switch between open windows in applications via Option-Tab, instead of using the built-in Command-Tab, which just switches between applications. I recently realized that, when Dashboard is active (F12), you can use Witch (Option-Tab) to select a window, and then you can do some things in this window while still having Dashboard active and frontmost.
For example, you can select a TextEdit window and write in it, jotting down some notes relative to something in a Dashboard widget. This will work as long as you don't click the mouse button. I've found this quite useful at times.
[robg adds: This could also be useful for selecting text in a background window and copying it (hold down Shift and use the arrow keys to select text, then press Command-C), for pasting into a Dashboard widget, if you forgot to copy it before you invoked Dashboard.]
