QuickLook is a feature in Leopard, which allows to view file content without opening it, directly from Finder.

Here is the list of additional plugins I have installed on my MacBook (to install simply copy.qlgenerator package into /Library/QuickLook or ~/Library/QuickLook):

1. BetterZipQL - view content of an archive (many archive types are supported)
2. FolderViewer - view content of selected folder
3. QLEnscript - colorized preview of source code files
4. ColorXML - colorized preview of XML files