Quick Look is a powerfull feature of Mac OS X which is activated when hitting space after file(s) selection; let’s see why:

1. Multiformat Support: can open an amusing number of file format including, but not limited to:
   PDFs, HTML, QuickTime readable media, plain text and RTF text documents, Apple Keynote, Pages and Numbers, ODF documents, Microsoft Word, Excel, and PowerPoint files (including OOXML), RAW camera images.
2. Plugins: support for additional formats can be achieved via use of 3rd party plug-ins.
3. Index Sheet: if you select multiple files before activating Quick Look, you’ll just be shown a preview of the first file. Clicking Index Sheet is just like using exposé, only with the previews rather than windows.
4. Slideshow Mode: when using Quick Look on multiple images, there is a play button at the bottom to start a slideshow.
5. Zooming: if you activate Quick Look on an image, hold down the Option key and you will notice that the cursor changes to a magnifying glass. Now, using the scroll wheel you can zoom in and out.
6. Keyboard Navigation: while you are previewing a file with Quick Look, you can still use the keyboard to navigate to other files. In column view this is easily done with the arrow keys. In the other views, use Cmd-DOWN and Cmd-UP to go into and out of folders.
7. Trashed Files: can preview trashed files without moving them out.

NOTE1: Quick Look has been introduced in Mac OS X Leopard and it is still present in Mac OS X Lion.

NOTE2: Quick Look is just enough to prefer Mac OS X over Windows! 😛