Windows only: Glass Notepad extends the Aero transparency effect to the entire editing window—a slick replacement for the built-in Notepad utility.
Glass Notepad works just about exactly the way you'd expect—it's simple and lightweight, contained in a single executable file, and doesn't have a lot of features—but it enables Aero transparency across the entire window. It's definitely not a choice for a power user, but those interested in really customising their Windows desktop will probably want to take a look.
Glass Notepad is a free download, requires Windows 7 or Vista with Aero enabled. For more transparent goodness, check out the Glass command prompt, the All-Glass Firefox theme, and the more productive Aero Geek Firefox customization.