Winamp has come along way, but since it was bought out by AOL it hasn’t really been innovating much. I got the latest version and they have this feature that lets it become transparent (which it has done for awhile) but what they have added to this is that it becomes fully opaque when you mouse over it and fades to transparent again when you move your mouse away.
This simple interaction makes the transparent feature useful (which it was really before so didn’t use it much) but is also an interesting idea for an interface design. You could have more than one thing/window transparent and layered on top of each other. As the document was further towards the back of the pile (i.e. say a group of journal entries stacked by date) they could be more/less transparent (need to experiment with opacity). Possibly flip/zoom though the layers with the mouse wheel and on the mouse over increase the opacity to full (or readable level?).