You can't get/use it until you start with Leopard. Basically, it will display most files as tall as your screen and you can read the first page of it, at least if it's an html, text, Word, etc. document. If it's an image, same thing, displayed as large as your monitor is tall. If it's a music file, it will play, just like in Tiger. The up/down arrow keys allow you to look through a list of images/songs/etc. very quickly and find he stuff you wish you'd never downloaded from the camera!
Movies can also be played, but they will only be as big as the format is natively. Also, there are controls for the movies in the window. For images, there are a couple of buttons. One that will zoom it as tall as possible, blanking out the menu bar and every thing else on the screen. A second one will allow you to add it to your iPhoto library.
And, if you select a group of files, there will be a button to show an 'index sheet' view, select the next/previous file and/or play each one for a short period of time (a built-in slide-show of whatever you have selected).
All the above is done by selecting something in a Finder window or the desktop and pressing the space bar. Simple, elegant, fast, free (if you have Leopard!). And did I mention it is simple?
And no other application is required, it's just part of the OS, Leopard, that is... I find it extremely useful.
