Using a combination of fixed positioning, relative positioning, and transparent /examples/images you can create a "secret message", which will only reveal itself if you scroll down and line up the letters. PSD included in the example which shows simple masking technique.
Thanks to some CSS positioning tricks, we can actually have two different page elements in two different areas of the page use the same anchor link and react to the same hover event. Pretty cool effect that uses nothing but pure CSS and degrades very well.
Parallax is very cool three-dimensional effect. "Closer" stars will appear to move faster while more "distant" stars move slower. This is achieved with transparent background /examples/images and percentage positioning. Downloads include Photoshop (.psd) file.