Widget Watch: Abracadabra
There are many ways to "hide" files on your Mac, and Dashboard Widget Abracadabra makes it drag-and-drop easy. Just grab the file you want hidden, invoke Dashboard and drop it onto the magic hat. Presto! It disappears from the Finder and Spotlight.
To retrieve your file, just flip the widget over and enter the password (you can even password protect this process, if you're super-paranoid). As tipster Lewis points out, it looks like this widget puts a "." in front of your filename to work its "magic."
Thanks, Lewis!