Sage Thumbs is an excellent utility for adding the ability to preview Photoshop PSD files in Windows Explorer. The thumbnail preview capability speeds up the workflow significantly in Photoshop, especially with multiple files in the same folder. It requires the Ghostscript Library for Adobe Illustrator, Postscript, and PDF files.
http://code.google.com/p/sagethumbs/
There is support for 162 file types via the included GFL Library.