That new button you see with an eyeball on it in the toolbar of Mac OS X 10.5 Leopard is the Quick Look/Slideshow button.

Use this feature to preview any file, photo, graphic, PDF, or whatever. An image appears immediately in front of you and no application opens. With the click of a mouse, the preview disappears.

Excerpted from Mac OS X 10.5 Leopard: Peachpit Learning Series