• /Macintosh HD/System/Library/QuickLook/
• /Macintosh HD/Library/QuickLook/
• /username/Library/QuickLook/ (if present)

By default, only the first directory listed above should have any plug-ins in it. The other directories should contain third-party plug-ins supplied by developers to support their applications. Apple's included plug-ins are the following:

• Audio.qlgenerator – Support for audio files (MP3, M4A, WAVE, MIDI, and so on)
• Clippings.qlgenerator – Clipboard text, image, and link clippings
• EPS.qlgenerator – PostScript and illustrator files
• iChat.qlgenerator – Saved iChat transcripts
• LocPDF.qlgenerator – Localized PDF documents
• Office.qlgenerator – Microsoft Office documents (Word, Excel, PowerPoint)
• Package.qlgenerator – Folders of contents displayed as individual files
• PDF.qlgenerator – Basic PDF files
• SceneKit.qlgenerator – Lion-specific "scenekit" 3D-rendering documents
• Security.qlgenerator – Keychains and certificates
• StandardBundles.qlgenerator – Application and preference pane bundles
• Text.qlgenerator – Plain and rich text documents
• Web.qlgenerator – HTML documents

http://www.quicklookplugins.com/

http://www.qlplugins.com/

http://www.mactricksandtips.com/2008/02/ultimate-list-of-quicklook-plugins.html