To understand the purpose of previews, please refer to the article How do the Mac OS X Leopard features Quick Look, Cover Flow and Spotlight disrupt HSM and file archiving?

To generate the preview images for Quick Look and Cover Flow, the Finder runs Quick Look Plugins called .qlgenerator plugins which are supplied by Apple or the software development companies that render previews for this express purpose.  Quick Look .qlgenerator plugins are installed each time you install new software and are stored in the System:Library:QuickLook folder.  Apple provide's development tools for creating and testing these plugins.

You can search for Quick Look .qlgenerator plugins on most software directories such as VersionTracker, MacUpdate and the blog Quicklookplugins.com.