One of the few applications built-in to OS X that seemed to have been forgotten since the release of Dashboard is Stickies (found under Applications in the Finder). Simply put, it's a collection of digital sticky notes for your screen. It's great for jotting down phone numbers, building a grocery list, and many more impromptu bits of information you come across throughout the day.

By default, the sticky notes are a solid block of color (yellow, blue, etc). You can, however, set them (individually) as translucent. This is useful for working on a note while a video plays in the background, or retyping text from another window behind the note.

To set a note as translucent, select it, then go to Note | Translucent Window. The result will give you something like the following:

Notice the top and bottom notes are normal (solid), while the middle note is set as translucent.